With me now re-reading The Peculiar Case of a Novels Villain and finding fanfiction about it - it half Temps me to make my own.
Especially now that my brain has formulated an idea of Demon!Allen, with an Actual Mage!Az + his familiar 'Alice' - which is basically spaned from MooMaow's "Missed" One-shot.
Which is like, Allen does die during a demon attack - but doesn't STAY dead. The Demon King has basically been doing necromancy, bringing humans that were killed by a specific Demon type back to like; The catch being, they come back as demons and are forced to work for the Demon king.
So the entire story follows a Demon Allen (who may have had his memories whipped) that's starting a rebellion under the Demon Kings nose, with help of a Mage he accidentally *MaGiCaLlY* bonded too and said Mage's goofy Black-magic familiar that telepathically talks to him.
Writting this here so I won't forget.