Transformed into a mane (demon)?

[Nachti]

Long ago I read a book where a being was transformed into a sentient mane and through luck it ascended to a higher ranked demon within the abyss. I just dont remember in what book that happened. It was either a known wizard or another demon that was killed.

[Laughing Octopus]

Mane in question:

[aaron22]

in a world of magic, this seems totally plausible. i would also imagine that some of these warlock pacts end in a similar manner. fighting the blood war.

[Maecius]

There's a lot of lore out there about this. The lore on the Fugue Plane, in particular, suggests that devils regularly recruit from the dead per Kelemvor's Agreement (note that Kelemvor is not a god in our timeline, but one might assume a similar agreement is in place for us):

Souls can reside in the city for up to a tenday before a divine servant comes to collect them, often completely clueless to the fact that they are dead. During that time, baatezu are allowed to inform souls of their state and bargain with them. Souls are offered the chance to become devils themselves, usually starting as a lemure but having the chance to advance through the devilish ranks, possibly even becoming a pit fiend. This is the main way baatezu propagate. The prospect of becoming a devil may seem abhorrent to good-aligned mortals but those who follow evil deities and those who fear what awaits them in the afterlife are much more likely to take up the offer.

With regards to demons in particular, the Fiendish Codex I has this to say about their "life cycle:"

The Demonomicon insists that all demonspawn begin as manes (larvae demons) or other lower-order fiends and, over the centuries, progress up a ladder of power and evil until they become dretches or rutterkins at the whim of a powerful nalfeshnee or demon prince.

Once these demonspawn join the hosts of the Abyss, they then slave away in servitude and fear, hoping to go unnoticed by their masters -- for notice means torture and pain -- while perversely longing for the spark required to transform them into a higher-order demon. [...]

[...] While the Black Scrolls of Ahm do not refute the Demonomicon's position on the order of demons, Tulket nor Arhm disagreed on two important points. First, he wrote that demonspawn could instantly become any level of demon, depending on the "desires" of the Abyss itself. Second, he asserted that the "ladder" of demonkind is more of a circle (though his picture of the demon hierarchy looks more like a drop of blood than a circle).