If you have a positive relation with the Demon Worshippers, you can rescue them from prisoner stacks. The higher your relation with them (and persuasion skill), the higher level demon units you can rescue.
Not sure if this still works or not, but when I played the game (4 years ago or so), Ialoch was captured by an enemy faction. I captured the castle he was imprisoned in, and he offered to join me as a companion or a general (I forget which.)
I think he might've respawned as an enemy general as well (so 2 of them going around), and the game bugged out if they met.