Archdevils

Archdevils are the Infernal pinnacle: near unkillable, with high reality altering powers, and armies of Demons and Devils. They're incredibly variable in appearance, only rarely even humanoid, and often embody concepts of the Third Dimension. Killing an Archdevil is impossible in The Instability and only sends them back home from the Third Dimension (which depending on the Archdevil's powers may be very useful or near useless). They also have extreme power in The Instability, with reality altering abilites, and diety level powers outside of The Instability. Few ever leave The Instability thankfully, as nearly every time one does it means something is about to happen on a galactic scale.

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Archdevils do not follow the same constraints as Demons or Devils: their powers are insanely variable and highly conceptual often. Archdevils have power closely linked to powers of death in the world: one may have powers of Exhaustion or Disease, another of Time. They have minimal wordly impact when in The Instability, but can still mess with fringe cases of their domains.
A good example of an Archdevil's power set is Inligulae, the Time Eater. The Time Eater is a massive worm that has powers over time, especially people beating time. They're one of the biggest reasons time travel hasn't been invented: anyone who gets anywhere close or somehow finds a way to travel through time gets killed by the Time Eater before they can do anything about it. The Time Eater can move between timelines, shape time, and manipulate the lifespan of humanoids. However, it is not fully evil, mostly a Lawful being, and has no real malevolence, just trying to finish what time began.