#glistener's guide to new phyrexia

[littjara-mirrorlake]

From my in-progress homebrew D&D 5e supplement, Plane Shift: Mirrodin/New Phyrexia: playable Myr!

They've been beloved in playtesting, with no fewer than three myr PCs appearing in the party over the course of a 3-year campaign. They are one of two new playable races in Plane Shift: New Phyrexia, along with the core-born Phyrexian.

Constructed Resilience and Sentry's Rest are abilities that previously appeared on the Warforged in Eberron: Rising from the Last War, and Regenerative Repair is a less restricting version of the ability Healing Machine from Astral Adventurer's Guide.

Text from the image under the cut!

Metallic, beak-headed myr inhabit Mirrodin, scampering at the feet of larger humanoids and largely considered below their attention. Few know of their true origin as creations of the mad wizard Memnarch, designed to be mechanized servants and his eyes across the plane. Following Memnarch’s fall, the myr found themselves with sapience and free will, though their core values of duty, community, and knowledge remain.

Myr Traits

Type. You are a Construct. You are also considered a myr for any prerequisite or effect that requires you to be a myr.

Ability Score Increase. Your Intelligence score increases by 2, and your Dexterity score increases by 1.

Age. As constructed creatures, myr don’t grow old in the traditional sense, and they are able to live indefinitely if well-maintained. You are immune to magical aging effects.

Size. Myr average about 3 feet tall. Your size is Small.

Speed. Your base walking speed is 25 feet.

Darkvision. Your constructed senses grant you superior vision in dark and dim conditions. You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern color in darkness, only shades of gray.

Constructed Resilience. You have resistance to poison damage and immunity to disease, and you have advantage on saving throws against being poisoned. You don’t need to eat, drink, or breathe. You also don’t need to sleep, and magic can’t put you to sleep.

Bonus Proficiencies. You gain proficiency in one skill and one tool of your choice. The tool you chose is integrated into your body and cannot be removed while you live.

Networked Minds. You can communicate telepathically with other myr within 120 feet of you.

Sentry’s Rest. When you take a long rest, you must spend at least six hours in an inactive, motionless state, rather than sleeping. In this state, you appear inert, but it doesn’t render you unconscious, and you can see and hear as normal.

Regenerative Repair. If the mending spell is cast on you, you can expend a hit die, roll it, and regain a number of hit points equal to the roll plus your Constitution modifier (minimum of 1 hit point). Spells such as cure wounds and spare the dying which restore hit points or preserve life, and normally don’t affect constructs, function as if you were a humanoid.

Languages. You can speak, read, and write Common and one other language of your choice.

[planar-echoes]

Phyrexia and the Vaultlord (Mirrodin) By Doug Beyer (9/8/10)

Its name is spoken in whispers. It is synonymous with death on a mass scale, relentless and merciless predation, and the corruption and debasement of life. It is civilization founded on genocide. It is evil in its purest, most insidious, most hideously effective form. Once the nemesis of Dominaria, it now sets its sights on a new world.

It is Phyrexia.

*****

Divide and grow. That was the first rule of any organism, especially one that had been created as a weapon. For what seemed an eternity, the oil had lain dormant, waiting to be unleashed upon a new world. The war for which it had been created had long since passed, but when a new pair of travelers came, it awoke again and followed them to this new, this pristine world.

Divide and grow. Divide and grow. That was the first rule. Divide and grow until the oil infused the entire world. There was time enough for contamination and control later. For now, it must simply divide and grow.

—The Moons of Mirrodin, prologue

******

The Phyrexian Invasion of Dominaria ended in defeat. Urza and his planeswalker allies devastated Phyrexia, pushing back its witch engines and machine priests with the power of the assembled Legacy. Phyrexia was destroyed, its plan to turn Dominaria into its new home world a failure. The heroes won.

 Karn's Tainted Heart

Karn, the keystone of the power of the Legacy Weapon artifacts, realized his destiny and obliterated Yawgmoth—and became a planeswalker in the process. He now carries within him the combined power of his creator Urza and all the artifacts of the Legacy.

With Yawgmoth destroyed, Phyrexia had no recourse, no retreat plan. The armies of Phyrexia fell. Dominaria became littered with the hollow shells left behind by the Phyrexian apocalypse.

Since its defeat, Phyrexia is no longer a place. But it is still a force in the Multiverse. Thanks to an innocuous quantity of strange black oil, Phyrexia lives—and grows.

 The Phyrexian Oil: Source of the Infection

The "glistening oil" is both a contagion and a means of colonization, a viral substance engineered to spread Phyrexian corruption wherever it travels. Without knowing it, Karn carried a trace of the Phyrexian oil within him, inside the Phyrexian heartstone granted to him long ago by Urza. Karn left traces of the oil in his travels from plane to plane, including the artificial world, the world Karn himself created: Mirrodin.

The glistening oil planted a virus on Mirrodin that has spread. Unknown to most Mirrans, Phyrexia is rebuilding itself, using their world as its host substrate. The metal structures and metal-infused inhabitants of Mirrodin create a perfect breeding ground for the spread of Phyrexian corruption. As the nascent civilization of Phyrexia expands in secret, it struggles to evolve a unified purpose, and the danger for the plane of Mirrodin grows.

 First Contact

Some Mirrans have begun to encounter horrible amalgams of necrotic flesh and corroded metal unlike anything seen on the plane before. They have no word for these creatures, but the creatures' destructiveness is clear, and the casualties have already been dramatic. The creatures' necrotech mimics forms found in nature—skeletons, teeth, viscera—yet their forms are a mockery of life. They are a race of killing machines, living to destroy, envelop, and replicate.

It is becoming clear to the innocent Mirrans that they face a new threat, something far, far worse than the power-mad Memnarch.

The Spread of Phyrexian Influence

Phyrexian oil-spreaders have sprung up unbidden in certain areas of Mirrodin, infecting the surrounding area with the glistening oil. Some Mirrans, such as the vedalken of the Quicksilver Sea, study the oil with curiosity, not understanding the danger lurking within. Traces of the oil have even begun to corrupt the beasts within the Tangle Forest, its treelike copper structures providing an unlikely haven for Phyrexian monstrosities.

But the growth of Phyrexia seems to be centered on the Mephidross, a vast, oily swamp on the surface of Mirrodin. Over the ever-present, churning drone of the necrogen chimneys can be heard the scraping moans of the nim: Mirrodin's relentless, ravenous zombies. The noxious necrogen gas transforms living tissue into the undead, exposing and corroding metal endoskeletons, turning the Dross's native Moriok people into horrifying, deadly fiends.

At the center of the Mephidross lies Ish Sah, the Vault of Whispers. The Vault encloses the black lacuna, the opening where the black sun once emerged from within Mirrodin's core. As a potent source of black mana, the black lacuna has guided and facilitated the evolution of Phyrexia. And one who has regained power over Ish Sah has learned how useful collaboration with the Phyrexians can be.

 Geth, Lord of the Vault

Geth was an undead warlord of Mirrodin since the time before the coming of the green sun, sadistic and power-greedy to the core. His head became severed from his body, but even in that state, the deathlessly decapitated Geth maintained his cruel need for power. After the elf Glissa defeated Memnarch, she disappeared, leaving Geth to trudge back to Ish Sah in the Mephidross. In his decapitated state, he struggled to regain power over his legions of nim zombies. But he encountered a new power, and negotiated a deal.

For reasons only they understand, the Phyrexians granted Geth a new, Phyrexian metal body, grafting his undead head to it. Geth's new host body is arachnoid, repulsive, and unnaturally strong—perfectly to Geth's liking. Geth expects to have to repay his newfound dark masters for their gift, but for now takes advantage of the sinister powers it has granted him, and monitors the Phyrexians' movements in the Mephidross.

With the Phyrexian necrotech supporting him, Geth quickly wiped out other challengers to the Vault of Whispers and reclaimed it as his own. He builds a new army of nim, preparing to conquer a wider dominion, but with the patience of the undead. He remains bunkered inside Ish Sah, controlling the black lacuna and learning secrets about the Phyrexian newcomers.

For now, Geth is content to watch and wait for the best time to strike. Perhaps no single being knows more about the Phyrexians' plans and tactics on Mirrodin than Geth. Perhaps no single being sees as clearly the danger that Mirrodin faces.

[littjara-mirrorlake]

Plane Shift: New Phyrexia Phyresis Rules 1.0

Contact with Phyrexian creatures and glistening oil can lead to phyresis, a special condition tracked in ten stages. Phyresis is not a disease, so immunity to disease cannot prevent a creature from being afflicted. Phyrexian creatures are immune to this condition.

A creature infected by phyresis experiences the effects of its current level and all those below.

Until level 5, greater restoration may remove a phyresis level from a targeted creature in addition to its other effects.

Every 24 hours, a character who has at least 1 phyresis level must roll a d20. On a roll equal to or less than their current phyresis level, they gain one level.

Phyresis Level Effects

1: No effect 2: No effect 3: Disadvantage on saving throws against being charmed or frightened by Phyrexian creatures 4: No new effect 5: Phyresis can no longer be removed by greater restoration 6: No new effect 7: Phyrexian language proficiency 8: Gain one augmentation for which you meet the prerequisites (detailed in a later installment) 9: No new effect 10: Incapacitated; begin compleation saving throws (see “A Sublime Transformation”) (Below the cut)

A Sublime Transformation

Most compleated adult Phyrexians, including player characters, are capable of compleating other creatures with sufficient ichor and time. A compleated creature retains its original type and racial features but gains the Phyrexian supertype. As a general rule, spells cast using Phyrexian mana that raise a target from the dead will return them compleated, if they were not already.

Much like being raised from the dead, the process of compleation is an exhausting ordeal that saps the energy of affected creatures. A newly compleated Phyrexian takes a -3 penalty to all attack rolls, saving throws, and ability checks. Each time the creature finishes a long rest, the penalty is reduced by 1 until it disappears.

Often, compleation is a process of optimization, reinforcing a creature's existing strengths. In that spirit, compleated player characters may increase one ability score above 20 by subtracting 2 from another ability score for each increase by 1 to the target score, to a maximum of 22. In addition, the compleated character gains one Phyrexian augmentation for which they meet the prerequisite.

Compleation is a unique opportunity for a player to re-work their character, extending to even class and subclass choices. Additionally, the mnemonic nature of glistening oil means that genetic material is not the only thing passed down from a Phyrexian to a creature they compleat. A newly compleated creature gains one skill proficiency possessed by the Phyrexian who compleated them.

Glistening oil carries the voice of Yawgmoth, who seeks to bend all to his whims. When you reach 10 phyresis levels, you must make a DC 16 Wisdom or Charisma saving throw each turn (your choice). The Phyrexian compleating you may grant you advantage on these saving throws. Successes and failures don't need to be consecutive; keep track of both until you collect three of a kind. When you roll a 1 on the d20, it counts as two failures; likewise, a 20 on the d20 counts as two successes. On your third success, you become compleated while retaining your previous memories and convictions. On your third failure, your bonds are altered to serve Phyrexia. You retain your base alignment and personality, but may suffer memory loss. Either way, you lose all phyresis levels and their effects.

Though powerful, the alteration of loyalties during compleation can be undone. Dispel magic or remove curse cast with a 7th-level slot or higher can restore one target creature to its former bonds if its mind had been altered in this way. You can target one additional creature for each slot level above 7th.

[littjara-mirrorlake]

With MagicCon: Chicago plans starting to be finalized on WotC's end, and some information due from me on my D&D work, it's time to lock in a (working) title.

Plane Shift: Mirrodin/New Phyrexia is now Glistener's Guide to New Phyrexia.

I'll go through and change some organization accordingly, mostly the tag I use to discuss the book on this blog.

(I would have wanted to make an alternate cover titled Goldwarden's Guide to Mirrodin, Scars block style, but I have a feeling that commissioning one cover will already be more than pricey enough.)

[littjara-mirrorlake]

one of the great trials of writing plane shift: mirrodin/new phyrexia is that it is a rpg rulebook not a biology textbook so i cannot go into granular excruciating specificity about biological processes like phyresis. even if i want to

[littjara-mirrorlake]

what's your favorite part of Plane Shift: New Phyrexia?

The way it allows me, and all players and DMs who use the book, to tell more nuanced stories about Phyrexians.

It was one of my main goals from the beginning to create a supplement where Phyrexians were as viable and diverse a player lineage as any other, capable of becoming beloved, morally complex characters. It is not a coincidence that in the book, I have completely abolished the alignment chart.

I am trying to make Plane Shift: Mirrodin/New Phyrexia compatible with both Mirran and Phyrexian-centered narratives, and as a setting guide, it should allow for either to be played out. However, I do push strongly not to portray Phyrexia as a plague or scourge to be eliminated (as the Mirran side and Magic story usually describes it), but as a dystopia. Phyrexians are not faceless monster stat-blocks but nuanced potential allies, enemies, and player characters whose relationship to Phyrexia-the-empire and recognition of its harms vary broadly.

The book is built assuming a mixed Mirran and Phyrexian party (though it's not required), and if/when I get around to commissioning cover art, it will reflect that. Of course, games can work with only Mirrans or only Phyrexians, but the focus on revolution and dystopia also makes it possible for them to work together cohesively and compellingly in a single adventuring group.

[littjara-mirrorlake]

D&D 5e Homebrew Lineage: Core-Born Phyrexian

This lineage is part of my in-progress homebrew supplement Plane Shift: Mirrodin/New Phyrexia for D&D 5e.

The core-born Phyrexian is the build-it-yourself choice out of the supplement's options, similarly to its fellow MtG crossover, the Simic hybrid. Included with this version are a short list of sample modifications, which will be expanded as time and playtesting go on. Many are duplicates of existing racials in 5e, but others will be original, such as PS:MNP's weirdest modification so far: Metamorph. Something to look forward to!

Though the stat block here states a set ASI (+2 Con/+1 any), I'm actually a strong proponent of rearranging ability scores as in Tasha's, and the pre-set ASI is really more flavor text than anything.

The text here alludes to the phyresis mechanic, which will be posted in the future. In short, taking necrotic damage from Phyrexians will cause a Con save with DC equal to the damage taken, and on a failed save, you will gain one phyresis level. Phyresis advances in ten stages, with compleation being the end result. The minimum DC of this save is 5. Uniquely, a natural 1 always fails, and a natural 20 always succeeds.

Text from the images under the cut. (It's quite a lot.)

Core-Born Phyrexian

Though Phyrexians’ most well-known method of reproduction is through the compleation of other creatures, fully Phyrexian offspring can also be grown in glistening oil, tended by vat-priests. Germs are most often born from the oil of a single Phyrexian parent and take after that parent’s morphological features, including the type and color of their metal plates. Such Phyrexians are known as “core-born”, even though the laboratories that create them have expanded all over the surface and interior of Mirrodin.

Type. You are a Humanoid. You are also considered a Phyrexian for any prerequisite or effect that requires you to be a Phyrexian.

Ability Score Increase. Your Constitution score increases by 2, and one other ability score of your choice increases by 1.

Age. Owing to your combined construction of flesh and metal, the Phyrexian definition of life is porous at best. Phyrexian beings have the potential to live indefinitely. You are immune to magical aging effects.

Size. As a player character, your size can be either Small or Medium. Either way, Phyrexians tend to be heavier than organic creatures of equivalent size.

Speed. Your base walking speed is 30 feet.

Darkvision. Your constructed senses grant you superior vision in dark and dim conditions. You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern color in darkness, only shades of gray.

Necrotic Resistance. The glistening oil flowing through your body protects you from the tainted attacks of your kind. You have resistance to necrotic damage.

Tainted Strike. Your claws are natural weapons, which you can use to make unarmed strikes. If you hit with them, you deal 1d4 + your Strength modifier of slashing damage, plus 1 additional necrotic damage, instead of the bludgeoning damage normal for an unarmed strike.

Phyrexian Modification. You begin with one modification (presented later in this document) as a result of the Phyrexian work done on your body, and you gain an additional one at 5th level.

Languages. You can speak, read, and write Phyrexian and one other language of your choice.

Phyrexian Modifications

These modifications are presented in alphabetical order along with their prerequisites and associated factions. They are intended for use both as-is and as inspiration for individually customized features. Modifications are gained at 1st and 5th level by the core-born race, but are also obtainable by any character who has Phyrexian work done on their body or gains 8 or more levels of phyresis.

Unless stated otherwise in its description, each modification can only be taken once and has no prerequisites.

At the DM’s discretion, a character may exchange one of their modifications for another by undergoing a surgical procedure. A character must still meet any prerequisites for a new modification they take this way.

Constructed Resilience Faction: Any Prerequisite: does not already have the Constructed Resilience feature

Even among Phyrexians, you are unusually mechanized, forfeiting the majority of your organic components for the resilience of a true machine. You have resistance to poison damage and immunity to disease, and you have advantage on saving throws against being poisoned. You don’t need to eat, drink, or breathe. You also don’t need to sleep, and magic can’t put you to sleep.

To gain the benefits of a long rest, you must spend all 8 hours staying still or doing light activity, such as keeping watch.

Extra Arms Faction: Any Prerequisite: does not already have the Extra Arms feature

You gain an extra pair of arms that function normally, with the following exceptions:

You can use a secondary arm to wield a weapon that has the light property, but you can’t use a secondary arm to wield other kinds of weapons.

You can’t wield a shield with a secondary arm.

Incendiary Breath Faction: Quiet Furnace

When you take the Attack action on your turn, you can replace one of your attacks with an exhalation of fire in a 15-foot cone. Each creature in that area must make a Dexterity saving throw (DC = 8 + your Constitution modifier + your proficiency bonus). On a failed save, the creature takes 1d10 fire damage. On a successful save, it takes half as much damage. This damage increases by 1d10 when you reach 5th level (2d10), 11th level (3d10), and 17th level (4d10).

You can use this ability a number of times equal to your proficiency bonus, and you regain all expended uses when you finish a long rest.

Necrogen Vents Faction: Steel Thanes Prerequisite: 5th level

As an action, you can release a poisonous aura of necrogen in a 10-foot-radius sphere centered on yourself. It lasts for 1 minute, moves with you, and ends early if you are incapacitated or die. Each other creature of your choice that starts its turn in the necrogen or enters it for the first time on its turn must succeed on a Constitution saving throw (DC 8 + your proficiency bonus + your Constitution modifier) or be poisoned until the beginning of its next turn. Creatures that don’t need to breathe automatically succeed on this saving throw.

Once you use this trait, you can’t use it again until you finish a short or long rest.

Reactive Spines Faction: Any

As a reaction when a creature you can see within 5 feet of you hits you with a melee attack, you can make metallic spines erupt from your body in retaliation, causing the attacker to take 1d6 piercing damage. This damage increases by 1d6 when you reach 5th level (2d6), 11th level (3d6), and 17th level (4d6).

You can use this reaction a number of times equal to your proficiency bonus, regaining all expended uses when you finish a long rest.

Reinforced Carapace Faction: Any Prerequisite: does not already have a racial bonus to AC, such as Iron Skin or Natural Armor

The metal of your body grows tougher, deflecting mortal blows. You gain a +1 bonus to your AC when you aren’t wearing heavy armor.

Resistance Faction: Any

You gain resistance to one of the following damage types of your choice: acid, fire, poison, psychic, or radiant.

Though any Phyrexian may receive this modification, each damage resistance is most commonly associated with a specific faction (see “Phyrexian Factions” earlier in this document).

Spider Climb Faction: Any Prerequisite: does not already have the Spider Climb feature

You gain a climbing speed equal to your walking speed, and you can climb walls and other difficult surfaces without needing to make an ability check. In addition, at 3rd level, you can move up, down, and across vertical surfaces and upside down along ceilings, while leaving your hands free.

[littjara-mirrorlake]

NPC Stat Block: Glissa Sunslayer (D&D 5e)

The elven warrior Glissa Sunseeker was once an exemplar of her people, named by Mirrodin's creator as a guardian of the plane. She defeated Memnarch but returned to a people that scorned her, believing she had betrayed them and caused the vanishing of their elders. Short of options, Glissa retreated into Mirrodin's core and was compleated by the praetor Vorinclex, who would become her closest companion. In the years before Phyrexia's emergence, Glissa founded the Vicious Swarm and covertly established a foothold in the Tangle. She is praetor of the Swarm in everything but name, and brutally drives forward its ideal of survival of the fittest.

Glissa's Traits

Ideal: "To forget thought and obey one's instincts is the only true way to live."

Bond: "Together with Vorinclex, I will bring about a new Phyrexia where strength is all that matters."

Flaw: "I scorn the nurturing of the weak. Anything less than savage predation is soft-hearted coddling."

One of Glissa's spells, corrosive gale, is new in Plane Shift: Mirrodin/New Phyrexia.

This is the first of the NPC stat blocks in my homebrew D&D 5e supplement, Plane Shift: Mirrodin/New Phyrexia, that I'm willing to call finished--though as with everything PS:MNP that I post, no wording is final and it is all subject to adjustment.

[littjara-mirrorlake]

Mox Opal

Wondrous item, very rare (requires attunement by a spellcaster)

While this opal is on your person, you can use an action to magically regain one expended spell slot. If the expended slot was of 6th level or higher, the new slot is 5th level. You can use the opal this way a number of times equal to the number of magic items you are currently attuned to, and it regains all expended uses daily at dawn.

Art by Volkan Baga

(This magic item is part of my in-progress supplement, Plane Shift: New Phyrexia)

[littjara-mirrorlake]

Current favorite item on the New Phyrexia trinket table I'm making

How did it get there? How did you find it??

[littjara-mirrorlake]

It's been a busy, tough time for us all, but my work on my projects is continuing and I'd love to talk about them more.

Ask me things about Plane Shift: Mirrodin/New Phyrexia!

[littjara-mirrorlake]

I just joined my first playtest session of Plane Shift: Mirrodin/New Phyrexia as a player, and it was an absolutely incredible experience!!

I played a vedalken aberrant mind sorcerer and got up to blue mage shenanigans against opposing Gitaxians... I got a natural 1 attack turned into a crit with almost max damage thanks to advantage from a party member... most wildly of all the corner case I'd been thinking of ACTUALLY HAPPENED: my character got their tenth phyresis level (I chose to start the test session with 7) while downed and making death saves. Note to self to make a comprehensive ruling about that!

[littjara-mirrorlake]

congratulations to the most fucked up and evil typeline I've written in plane shift: mirrodin/new phyrexia so far

[littjara-mirrorlake]

Plane Shift: Mirrodin/New Phyrexia contains a number of new spells and magic items inspired directly by MtG cards. Here are two of them!

Corrosive Gale appears on the factional spell list for the Vicious Swarm, and Rout likewise appears for the Machine Orthodoxy.

Text under the cut!

Corrosive Gale 3rd-level conjuration Casting Time: 1 action Range: 120 feet Components: V, S, M (a drop of acid) Duration: Concentration, up to 1 minute

You conjure a whirling gale of acid in a 10-foot-radius, 60-foot-high cylinder centered on a point within range. When a creature enters the spell’s area for the first time on a turn or starts its turn there, it must make a Dexterity saving throw. It takes 4d8 acid damage on a failed save, or half as much on a successful one. Creatures with flying speeds make this saving throw with disadvantage.

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the damage increases by 1d8 for each spell level above 3rd.

Spell Lists: Artificer, druid

Rout 4th-level evocation Casting Time: 1 action Range: Self (30-foot radius) Components: V Duration: Instantaneous

A flash of blinding radiance erupts from you. Each creature you choose within 30 feet of you must succeed on a Constitution saving throw or take 7d6 radiant or thunder damage (your choice) and be pushed 10 feet away from you. A creature that succeeds on its saving throw takes half as much damage and isn’t pushed.

At Higher Levels. When you cast this spell using a spell slot of 5th level or higher, the damage increases by 1d6 for each spell level above 5th.

Spell Lists: Cleric, paladin

[littjara-mirrorlake]

As much as I enjoy DMing with Plane Shift: Mirrodin/New Phyrexia, I'd love if more people did so as well! It accomplishes playtesting from the perspective of someone who isn't me, which helps a lot!

I'd be more than happy to share materials and discuss games with those interested :)

[littjara-mirrorlake]

what's the final solution you came with the phyrexian mana?

The solutions I have are far from final, as I'm still deep in development and playtesting stages, but my current solution is to make ichor magic unlockable by a feat and provide a few hopefully not overpowered options, similarly to metamagic.

Here's some text as it stands right now:

You can only use one Ichor Magic option on a spell when you cast it. Each option specifies a hit die cost. You pay hit die costs by rolling that many hit dice without adding your Constitution modifier and taking necrotic damage equal to the roll, which cannot be reduced or prevented in any way. The hit dice you roll this way are expended. You have advantage on Constitution saving throws made to concentrate on any spell you cast using ichor magic. Arcane Amplification. You may heighten the effects of a spell by expending hit dice during its casting. The spell counts as one level higher for each hit die you expend. You can amplify a spell this way up to a maximum level equal to your highest spell slot plus one. For example, if you are a 3rd-level cleric and cast cure wounds with a 1st-level spell slot, you can expend 2 hit dice to make the spell’s effects count as if it were 3rd level. Ichorous Bane. When you cast a spell that causes one or more creatures to make saving throws to resist its effects, you may expend one hit die to give Phyrexian creatures disadvantage on saving throws they make against the spell until the beginning of your next turn. You may prolong this effect on each of your turns by expending 1 additional hit die. Visceral Focus. When you cast a spell requiring a costly material component, you may forgo the component by expending 1 hit die per 100 gp of the material cost. If the component would be consumed by the spell, the hit points you lose in the casting are subtracted from your hit point maximum until you finish a long rest.

The exact architecture of ichor magic is open to change, of course, but I'm invested in the idea of using hit dice as currency, both because they introduce a natural limiter (you can't spend more hit dice than you have, and they are a real cost due to HP loss and attrition, forcing you to truly consider your priorities) and they make hit die numbers on NPCs meaningful, which they typically aren't in 5e from my experience. Also, ichor magic provides Phyrexian "typal hate," which I'm adding because I have been conscious of the strong bias towards Phyrexian options and power level in the supplement.

Something I'm definitely seeking to avoid is creating spell slots entirely out of hit dice.

Honestly, the idea of "blood casting" has just always been compelling to me and this is the perfect setting to try and make it work. Legacy versions of ichor casting have lent themselves to some very potent dramatic moments!