I think it's random but maybe completing some quest gives opportunity, don't know. But I find Demons more playable than Archdemons (obtained them few times by Demon Gate) especially my favorite combo now (since I divorced with Feanora and got rid of Royal Snakes):
-Scoffer Imps
-Imps
-Demons
-Xeona wife
due to tactics set both imp stacks as near enemy as possible, cast mass haste, put pentagram on ground with demons (imps get +3 morale) and hit with imps using their natural ability to strike and return. Next turn you can give fit of energy to one of them for throwing fireball at the end of turn. This setting needs tank like griffins or horsemen (I use horsemen due to their high initiative). Also Black Dragons are annoying but with help od dryads (put to sleep other troops and finish Dragons) their not very painless. OFC 2x Dragon Slayer Sword works very, very fine against Dragons
