Somewhere between Satan and The Legions of Hell are these guys: Maybe the setting doesn't want to deal with a single representative of all evil, maybe they felt dividing the stuff up makes for more fun intra-demon politicking. Maybe they felt it allowed for more specialization and character, or maybe it's easier to deal with underlings than Old Scratch himself.
Thus there are these guys: usually with names taken from apocrypha, the Ars Goetia or The Divine Comedy, these guys can be described as Almost-Satan, for when the real deal isn't as fun to use or needs a crew of big-name minions and agents between himself and the demonic hordes. They're often given fancy titles and pretty much do whatever Satan would have done had he been present, although it's also common for them to be given specific purviews, interests or schemes — theming each around one of the Seven Deadly Sins is a popular choice. Also, if you actually kill one or stop their schemes, it's still an accomplishment, but it's hardly a collapse of the demonic system. There will always be another to replace the one that falls. Perhaps even the killer. Keep in mind, killing one is a lot harder than it sounds, because miraculously driving one's Hit Points to zero usually involves Fighting a Shadow.
