Ideas needed for BBEG using devils

[Unoriginal]

If we want to honor the D&D tradition of splitting hairs of various infernal creatures:

The difference between demons (such as succubi)

Succubi aren't Demons in 5e.

Since newer editions are bad at explaining this, let's harken back to old times:

The chief conflict between Chaos and Law is that of the individual versus large, organized groups. Hence, when it comes to extremely Chaotic Evil creatures such a demons, even the lowliest them of thinks of itself as the center of the universe - and any suggestion otherwise is offensive to them in a way that makes them want to hurt somebody. They won't really obey anyone unless it pleases them personally or somebody's forcing them.

5e does actually an incredible job of explaining that. Especially in the Mordenkainen's Tome of Foes, which has the best demonstration of that ever, IMO.

Lawful Evil creatures, such as devils, don't think like this. They believe there is a hierarchy to the world, with the Better People deserving to be on top and the Inferior People on the bottom. As corollary, lower-level devils don't need a reason to obey the usurper king other than that he's on their side - he's the Chosen One who will bring Hell on Earth for greater glory of Devilkind. They will happily bend the knee to their king it means the rest of creation will be made to bend the knee before them.

This part isn't really accurate, though. At least not for this edition.

*Some* Devils are genuinely servile, sycophantic, or even personally loyal to their superiors, but most of them aren't.

The average Devil thinks of themselves as one of the Better People deserving to be on top, and that reality doesn't reflect that is nothing but a temporary embarrassement they will correct. They obey their superiors due to a mix of This Is How Things Are Done, legal obligations they can't dodge, fear of punishment, and thinking obeying now will give them opportunities later if they aren't already using the opportunity to try and advance themselves right now. In fact, most Devils are actively working to betray their hierarchical superior and take their place.

So no, the odds of the usurper king having *several* Devils who would happily bend the knee to him are quite low. The odds of having Devils *pretend* doing it while taking the king to the cleaners or moving their own pawns in the shadows is quite high, however.