#your body vessel for a spirit of will! like ysayle! or ilberd! or zenos!

[nemesis-is-my-middle-name]

the first introduction we have to primals, at the start of the game, tells us that they're:

gods

summoned by the beast tribes

using crystals aspected to the deity's element

and a specific summoning ritual

that are unilaterally aether-hungry and destructive

and then almost every primal after that spends its time going no, actually, that's a really narrow and kind of inaccurate definition. shiva and shinryu demonstrate that anyone can summon a primal, if they know how to and they have the ingredients. the game also implies, though doesn't really outright state, that primals aren't actually the gods they claim to be and don't even really have. a single coherent consciousness. they're more of an influx of aetheric energy defined by the circumstances of their summoning. as evidenced by the summoning of bismarck acting nothing like how bismarck would rightly act, and hraesvelgr flat-out telling ysayle that she's never actually communicated with the real shiva, she's just tricked herself into thinking she did, and then also the fact that when the kobold child gets in the way of titan's summoning it throws the entire consciousness out of whack.

the final summoning of garuda, and then shinryu and tsukuyomi show that you don't even need specific belief in the primal you're summoning for it to work. any kind of fervent faith will do, though it's arguably easier to use a deity as a focus because if you don't know in advance that it's possible, "i'm going to create a big monster with my mind" is sort of a difficult jump. you don't even need it to be the belief of people in your immediate vicinity, because yotsuyu uses the history of the mirror she's given to do it all by herself.

also, you can use yourself as a vessel for a summoning, and still retain your consciousness and survive the primal's dissolution, as shown by shiva, and both shinryu summonings, and tsukuyomi. and then there's susano: a kami who arguably effects his own summoning once you bring the three treasures together. and all of those circumstances, along with ramuh, show that primals, while needing a constant influx of aether to sustain themselves, are capable of retaining a consciousness beyond hunger.

so okay, to recap, our new list of primal understanding is:

they're beings formed of a massive concentration of aether

created by some intangible force of united belief

and a single focal point with a desire to create a primal

so to jump back to my original point, the climactic emet-selch fight. at the point right before the battle, the protag is like, beat. the entire party has been comprehensively stomped, and the WoD can't even muster the strength to take another step, consumed as they are by the light within that's finally winning against them. y'know, the massive amount of light aether that was enough to create five lightwardens and change the entire sky.

also, as the talos-building and then your departure from the crystarium showed, at that point, nearly every single person in norvrandt believes with their whole heart in the Warrior of Darkness. maybe not you, specifically, not everyone, but they believe in the nigh-mythic hero bringing back the night, and they believe in the person who's gone toe-to-toe with the lightwardens, beings who can't even die without perpetuating themselves and destroying their attacker, and everyone in novrandt wants you to win, motherfucker. they believe in you.

so we've got aether. and we've got faith. and we've got a single focal point to channel both those things. the final rub is element. the game isn't really... 100% clear whether the primals have to match the elemental energy you use to summon them, (or if it is they care little enough about it that i don't remember), but it definitely influences them, and if you want a specific thing to be created you're probably gonna want to go with the elements that match it. also, dumping a bunch of light into most things fucks it up, like, a lot, as the sin eaters/lightwardens and the whole of the empty demonstrate. the only exception to this that you're aware of at this point is the WoD, who was already primed and able to hold a whole bunch of light via hydaelyn's blessing. (true, you fell apart at the end there, but you were doing really good for three or four wardens, which is three or four more than anyone else can claim!) so even if you do have everything else you need for a summoning, you're gonna need a focus that can actually sustain it without turning into a mad monster.

and what do you have but ardbert, the spirit shade tied to your being, who's been following you around inarguably extant but unable to act without a body. a mother fucking Warrior of Light.

do you see what i'm saying!! you summon him!! you pour all that Light into his spirit, taking everyone's faith in you and turning it into a net for him, using your single fractured body and soul as a focus, passing the torch on to someone who can hold out against the light for a little bit longer. just long enough to see the job done. you bring the fallen hero of the first back for one last dance.

it's arguable that this is actually exactly what happens and it just would've been really fucking cumbersome to show in a cutscene but if shb ended with the WoD summoning ardbert like a primal it would have been a really epic and perfect climax of all the stuff they've been building up about primals throughout the rest of the game