Nimicus created Hell to be his version of Heaven, and he created demons as his answer to Primus's angels. The demons were ferocious, beautiful, proud, and passionate. But the demons were like Nimicus, and they turned against their creator just as Nimicus had turned against his own. They defied Nimicus, and, when he tried to force them to obey him, they rose up and fought back. Nimicus beat the demons into submission, but Nimicus saw that they would never give him the loyalty he wanted.
So Nimicus created devils. He created into them an obedient nature, and they serve him loyally to this day.
Demons despise devils as feckless lackeys. Devils hate demons as vile traitors.
Demons are proud to have been created by Nimicus, the "New God." Devils are proud to have been created by the "New God," as well, and also proud to have Nimicus's favor. But demons claim that Nimicus secretly likes them best because they are free rather than servile.
Each devil has six lesser devils that report to it, and each devil is one of six of its own kind that owe allegiance to a devil immediately their superior. This goes all the way up to the six pit fiends who report directly to Nimicus.
Since devils are reliable, Nimicus has, through the millennia, sold off some of them as slaves to various powerful beings. They are loyal without question to the masters that Nimicus assigns them to. Typically, a devil traded off as a slave takes with it the six devils that report to it, and the 36 devils that report to them, etc.
Devils were shock troops in the assault that leveled Holystone and killed Vanashon the Uniter. They may have been assigned out by Nimicus directly, but more likely these were assets owned by some lesser being.
Holy clerics can't summon devils without becoming corrupt. Whether an act corrupts one does not depend on the person's intent but on the act itself. Even if a cleric summoned a devil for the best reasons, the act would still corrupt the cleric.
Holy wizards, however, are trained to cast unholy spells and not be corrupted by doing so. Thus, holy wizards often summon devils. These are devils that, over the millennia, the forces of Heaven have captured and compelled into service.
Monk, paladin, cleric, psychic warrior, and rogue/monk up front; wizard hanging out in back.