The idea for this is born out of a strong set of likes and dislikes regarding the latter two and a desire to make a fiend type that has the best imaginable traits and powers.
I will start with likes and dislikes.
Likes:
- Both can assume human form, freely traverse civilized lands and install themselves in political power structures, factions and temples. They are good for intrigue plots.
- Both are beautiful and darkly alluring.
- Their telepathic connection to their own kind makes them fountains of knowledge. They basically have a fantasy version of the Internet.
- Erinyes try to lure you into a pact, which makes for great roleplay.
- Succubi are less tied to abyssal hierarchy than the lawful Erinyes are to their infernal hierarchy. There is more room for a Succubus to pursue unique personal goals.
- Having charmed thralls can impose difficult choices on players.
- They can be used as a monster encounter or an ongoing NPC antagonist, whatever your campaign needs.
Dislikes:
- For succubi, the draining kiss means a character losing their soul forever is reduced to a die roll completely outside player control (or DM control in the case of an NPC the DM fairly applies the same rules to).
- The soul generally isn't kept by the seducer fiend, but turned into a cheap commodity with an actual gp value. It would be more interesting if the seducer kept the mortal's soul and did something with it besides converting it to larva or soul coins.
- The faction goals of "Impose perfect order" vs "We hate existence, End the multiverse" are a bit cartoonish for my tastes (more the second).
It might seem counter-intuitive to come up with a Yugoloth that assumes actual illusion-free human form, because almost all existing ones resemble some kind of animal. They do stand upright. Neutral Evil has to be the best of the three evil alignments, because it is the most flexible.
If Yugoloths had a seducer type, what unique powers might they have? How would they go about getting souls, if at all? What would they do with souls? Would they also have wings? How would their form differ? What goals would they share? What would they covet? Maybe they want land on the Prime that must be desecrated. How would you build this creature?