The Dark Eight only answer to Asmodeus, Lord of the Ninth. They spend one day a year in with him. They are otherwise in surpreme charge of the Blood War.
Bel was appointed to command the armies of Avernus by the Dark Eight. He used that power to launch a coup against the ex Lord of the First. The other 7 Lords insist that he keep both jobs so that he is to busy to plot against them.
I've had a couple ideas for NPC's, and I'd like to know what exactly the guidelines to a lesser God taking one of their followers as an avatar are, or if it's even possible without the God losing his position?
Avatars are a portion that the diety seperates from themselves for a time. However a god can empower a follower or minion as a Proxy. They usually don't make many because they must place a divine rank in each for as long as the diety keeps them as a proxy.
Demons(ce) advance because they convince other demons that they are more powerful and (in the outer planes, belief determines reality) as more come to belive in their power, they become more powerful types.
Devils(le) have a specific and rigerous method of determining which ones are qualified to advance and what trials they must endure to advance and to what type.