Silence! I agree to a degree that the Demons which may control a civilization should not be overtly known to rule, they should puppet the leader.
A demon comes in contact with Elven hunter Ditxy, but does not kill it, chooses to drive it mad, enslave it, then send it back to it's civilization as a puppet. The puppet Ditxy becomes a great hero, after fighting against a great demon, who strangely looks just like the one who corrupted Ditxy. Ditxy becomes a big player in the Elven civilization, and prompts greater specialization in anti-demonology, through the mastery of demonology... And in come the demonic possessions, the blood pacts, the dark magics, sacrifices. Quickly these Wild Elves have become Hellish Elves, with red glowing eyes and great arcane power, but all being directly controlled by Ditxy's master, while Ditxy looks like the second in command. Then comes a horror, the demon returns with a bunch of Goblins it threatened into attacking, a huge battle ensues, the Druids are slaughtered in the ensuing madness and Ditxy takes control as the new Lord of the Hellish Elves by using his amazing power to drive back the demon.
Or at least, that's one history I thought would be neat to uncover.