Demon who grows, and grows, and grows, and none can say when they will cease.
The succubyzant is not the final instar, of course.
After ages of rampage, torment, and terrorisation, the byzant moults: not splitting open its skin to disgorge the next stage, but swelling into a hard, fleshy sphere and then splitting open space to moult through, leaving behind the hollow petrified geode-skin of its former body.
The final form of the succubus is the succubehemoth.
Hatched into a new, pocket dimension of its very own, the behemoth begins as a ball of infernal flesh no larger than the hollowed skin it just evacuated; but — unconstrained now by the demands of independent locomotion or inconvenient physics — it grows. There is no known upper bound on the size of a succubehemoth; existing in solipsistic universes unto themselves, they are wrapped in starless skies which exist solely to contain them, and they grow vast.
It is speculated that every Hellrealm is, in fact, the inhospitable planet-sized bulk of some ancient behemoth, grown large enough to exhibit appreciable gravity and weather systems on its skin. This implies a chronology to the Hellrealms — the least hospitable being the eldest.
This final metamorphosis may be some relief to the targets of the byzant instar's obsessions; abandoned, finally, to die. Less so to anyone who somehow finds themself on the surface of a hate-fuelled psychic planetoid, though the attentions of the behemoth form seem to become ever less personal with size, age, and increased surface population.