#especially no important peons who can show up with a crest of seiros

[randomnameless]

Does that mean Mercie and Supreme Leader could be distant cousins potentially? Guess it might be an idea in that 10000 year lore.

You have no idea how many cans of worm this AU opens anon -

AU where Dimi marries Mercie, and the resulting Blaiddyd heir has... a crest of Seiros :(

No one understands why, maybe it's a Hresvelg curse for destroying Adrestia?

With time, the Crest of Seiros is known as the Crest of Hresvelg (to Rhea's chagrin) and is now seen, in the Kingdom's folk legends, as a curse that befell humanity when it became too greedy.

AU where it's Papa Martritz's line that is a branch line from the Hresvelg House -

Ionius not managing to roll for crested heirs with his different wives jumped on the occasion to destroy House Martritz (even if it has a heir! in the over, but it exists still!) and put an end to eventual pretenders to the Imperial throne, if his "lady friends" can't give him an adequate crested heir :(

And even if the baby survives, it has a chance of not getting a crest of Seiros, and by virtue of not being a noble born baby, people might believe it's just a random bastard and not a legitimate threat to his dynasty

(Imagine if Baron Bartels got his seiros crested bby with Mercie though - hopefully, Emile made sure that plan would never see fruition!)

Ultimate combination AU : Mercie and Emile can tolerate Zanado fruits

Just like Constance!

Somehow, before being turned in a pincushion, lizard!Lycaon befriended Lamine's 9th daughter, who returned North when he "suddenly died" and Hildegard I (who descends from Willy's human children, the ones he got when he had his crest but from human women!) took his succession.

The resulting kid had no pointy ears but was "human passing" enough (only 1/4th Nabatean!), then said kid got kids - sometimes with a crest of Seiros, sometimes with the crest of Lamine - and their kids got kids - and they ended up in Adrestia, becoming Mercie's mom family.

Mercie will notice something strange because her parents recipe (that was influenced by her mom's cooking) always featured putting slices of fruit in the shape of a star on his various cupcakes - but then, those disgusting fruits Flayn, Seteth and even Lady Rhea nibble on occasion, when sliced, look like stars ? Granted, she never tried to eat any since everyone knows they're inedible! She tried to put a slice on a cupcake and ate it, to Annette and Dedue's surprise, she didn't threw up like everyone else, nor collapsed, even if Annette scolded her "Mercie, what are you even doing?! Those things can't be eaten!"

Overhearing Mercie's recipe history, Seteth frowns like never before - Lycaon used to hold hands with a scion of Lamine who joined their side (he told Seiros the Warrior to watch out because he was too young to hold hands, she ignored him, again!).

This will make Billy start to question themselves, if Rhea considered Mother like her daughter, and Mercedes is the child of the child of the child of the child (etc) of Rhea's son, is Mercedes their sort of nephew?