So I'm pretty sure we are all aware of the Fallen Angel trope and it's relevance to D&D. Four of the Lords of the Nine Hells are said to be fallen angels (Zariel, Dispater, Mephistopholes, and Asmodeus himself) as well as other archdevils (Baalzebul and Moloch) as well as an entire caste of the infernal heirarchy (Erinyes going by 3.5 lore that hasn't been contradicted since to my knowledge). I'm not aware of any Demons and Yugoloths that are fallen angels (or celestials in general), but if there was, I wouldn't be surprised. You can also count evil Empyreans as "fallen angels/celestials" and there is the Radiant Idols in Eberron that I think count (they even fell because they thought they were better than the gods, can't get any more fallen angel than that).

Yet, with all those examples of fallen angels, there are very few examples of Ascended Demons in D&D history and lore. There's a few examples of good members of traditionally evil races, the Mind Flayer Monk from 3.5 Book of Exalted Deeds as an example, as well as everyone's favorite punching bag Drizzt. But, even with those examples, there are very few examples of fiends that have managed to beat past their infernal nature and become good guys. The only one I can think of where the change stuck is Eludecia, a succubus paladin. There is a Bearded Devil in Decent into Avernes that is Chaotic Good, but that's because they have brain damage, and if you heal that, they go right back to Lawful Evil. I just wonder why there are more cases of fallen celestials then there are ascended fiends in the D&D world.

There is an answer to this question in the 5e PHB which says of devils that " A devil does not choose to be lawful evil, and it doesn't tend toward lawful evil, but rather it is lawful evil in its essence. If it somehow ceased to be lawful evil, it would cease to be a devil." (which I am reading as "that devil ceases to exist") Which is a fine explanation as to why there are no ascended fiends (assuming the same applies to Demons and Yugoloths, and there is no reason to not assume that). This does raise another question though, why does the same not apply to celestials? If a Devil that ceases to be Lawful Evil cease to be a devil why wouldn't a Planetar that ceased to be Lawful Good cease to exist? Is there something about Evil as a force that makes it, in essence, more powerful than Good as a force?

Basically, my question is this: Why are there so few Ascended Fiends in D&D lore compared to Fallen Celestials? Is it just not as interesting of a story? Is there something about Good and Evil as forces that prevent ascended fiends? Did the writers just never think of it? I'm interested to hear what other people have to say about this dilemma, or maybe you can give me more examples of ascended fiends in D&D that makes this whole thesis absolutely worthless, that's interesting too. (I know of Azuth being the old Lord of the Nine in Forgotten Realms or something, and while that's really cool and neat, it doesn't really fit IMO because Azuth is Lawful Neutral and I, personally am looking for Evil to Good examples)