What is the in-universe explanation for why succubi, who were demons, became devils?

Question

The Forgotten Realms wiki page on succubi tell us (specifically in footnote 1) that in 1e, 2e and 3.Xe¹, succubi were chaotic evil demons, but then were retconned to be lawful evil² devils in 4e and have now just been made into generic neutral evil "fiends" in 5e, presumably in an attempt to avoid contradicting any previous editions' lore.

Unlike with the Shadar-kai, I believe there was supposed to be some kind of canonical in-universe lore reason as to why these demons became devils. What was that reason? I assume it appears in some 4e material somewhere? I'm only really familiar with 5e material...

This question used to have another part to it, but that been split out into its own question; see: What is the in-universe explanation for why succubi, who were demons, became "neutral evil fiends" in 5e?

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¹ _{Actually, the footnote on the Forgotten Realms wiki page only says 3e, but I know it was still true in 3.5e because of Neverwinter Nights 2, which was a video game based on 3.5e. In this game they were considered demons, which is incidentally my introduction to D&D and why I consider succubi being demons to be what they "should" be.}

² _{I say "lawful evil", because that's what a devil's alignment is, but I'm aware that 4e changed the alignment system, so it might not be so accurate to claim they were "lawful evil" in 4e, but at the very least, in the context of D&D overall, they would have been considered lawful evil all the time they were considered to be devils.}

Answer 1

They're in a different cosmology.

Wizards Presents: Worlds and Monsters, p.66-67 and 73, establishes the behind-the-scenes reasoning: among many lore changes introduced in 4e, designers redefined demons as monstrous and devils as humanoid, which required succubi to become devils. Chris Perkins writes:

The succubus succubus, for example, is the D&D game's iconic seductress, and seduction is very much in keeping with the behaviour of devils. The succubus also hews more closely to the general description put forward for devils, which is to say that she's humanoid with some monstrous features (horns, tail, and leathery wings), and not a monstrous creature with some or no humanoid features (which is the barest physical definition of a demon).

D&D 4th edition is a new cosmology entirely. Page 17 of Worlds and Monsters declares "The Great Wheel is dead." It's not that the people of Greyhawk or Planescape are wondering why succubi became Lawful; rather, 4e is set in a different cosmology where succubi have always been Lawful.

In the Forgotten Realms

More complexity arises in the Forgotten Realms, where, canonically, the same continuous world has progressed through each edition of D&D. The transition of the Forgotten Realms from 3e to 4e was largely explained by the Spellplague, which changed the cosmology dramatically. The Forgotten Realms wiki notes:

Most of the Outer Planes were either destroyed or merged with others, creating the Astral dominions. Entirely new Astral dominions also arose because of the Spellplague.

With his new found godly power, Asmodeus took advantage of the planar instability to hurl the Abyss to the Elemental Chaos in an attempt to end the Blood War. Asmodeus failed, however, as throwing the Abyss into the Chaos did not stop the Blood War, although it was stalled by a hundred years.

The Forgotten Realms novel Fire in the Blood explains that Asmodeus' actions in the Spellplague are believed to be responsible for the change in the succubus' loyalties:

"The succubi are our sisters-at-arms," Lorcan said, repeating the phrase like a catechism. "Asmodeus freed them from the turmoil of the Abyss and found them their true place among us. And anyway," he added, " I doubt strongly there are any succubi in Stygia."

Brimstone Angels describes that the succubi defected to Asmodeus:

That mad, demon spark, as far as Lorcan and most of the Hells were concerned, still lingered. You could see it in their eyes. It didn't matter if they'd turned traitor just as Asmodeus rose to godhood, ceding their blood and their offspring's blood to the lord of the Ninth's control, and—the rumors went—giving Asmodeus the last bit of power he needed to hurl the Abyss to the very farthest reaches of the Elemental Chaos, ending the war for good.

Lorcan had his doubts about that—everyone claimed to have been the lynchpin of Asmodeus's ascension. Sycophants, all of them.

Answer 2

Sadly, there is no in-universe explanation for the recategorization. There's a story presented in Dragon Magazine 417 but it's purely retcon. The real world reason is that it was a result of "demon" and "devil" being more clearly defined.

Demons are creatures corrupted by the Abyss that thrive on raw chaos and destruction. Devils are fallen servants of the gods that employ temptation, manipulation, and seduction in hopes of corrupting, and ultimately ruling, the universe rather than destroying it. Because of this redefinition, some monsters were re-categorized. The most prominent among these was the succubus, which no longer fit the mold as a demon.

Answer 3

Actually there's a pretty simple yet very hand wavy and slightly annoying answer as to why they became devils. If you read the Brimstone Angel's series it explains it fairly well. Basically, the Succubi were aware that Asmodeus was going to steal the divine spark of Azuth after he fell into Baator during the spellplauge, so they more or less defected, and no one can confirm this, but according to some devils, the succubi defecting was somehow important and helped asmodeus make the final step from demigod to full on deity, tho no one can be sure of that. Even in the Brimstone Angel's series they make it fairly clear that the Succubi might be "devils" now, but they also still have the same chaotic force that drives demons deep down inside. Malconthet, Queen of the Succubi, was playing both sides the entire time more or less, being Demogorgons consort and Asmodeus' envoy at the exact same time in 1400s DR

Answer 4

Perhaps the authors of the previous editions in-universe texts were Clueless.

You could just write off previous material as being the result of scholars from the Prime Material plane who didn't know how the Outer Planes really worked. In Planescape, the term for individuals from the Prime Material was Clueless, and I believe that this may have been in part an attempt at explaining ADnD's retcons of the planes and their inhabitants (e.g. renaming the devils and demons as Baatezu and Tanarri during the 1e to 2e transition to appease the ongoing moral panic at the time), as well as its own retcons regarding the names and structure of the Outer Planes.

In the case of the 5e books, this is certainly true, because they're written by in-universe characters native to the Prime Material Plane - Volo's Guide to Monsters was written by a bard from Faerun, Xanathar's Guide to Everything was written by a Faerunian beholder, and Mordenkainen's Tome of Foes was written by Mordenkainen, an archmage from Greyhawk.

Additionally, there was a short story in an old issue of Dragon that indicated that Ed Greenwood (the creator of the Forgotten Realms setting) was actually himself a powerful wizard who regularly had meetings with other archmages from the settings of ADnD, which would mean that the officially-published books themselves can become viewed with suspicion as being "in-universe" documents written by in-universe versions of their actual authors on an in-universe version of Earth as a result of communication with individuals from other planes.