Right, I have absolutely no idea why you would ever want to do this, but... Meh.
Right, you need to be a Warlock for this.
Go to Zangarmarsh. To the far south of the Cenarion Expedition exists a cave with Mushroom Men and Elementals. Inside the Cave, engage a elemental and takes his HP down to about 20-30 %. Cast Curse Of Doom on him and then banish / fear lock him. If you are lucky, Curse Of Doom will Procc and a Doom Guard will spawn.
Cast Enslave Demon on the Doom Guard (Curse Of Shadow's first if you wish the enslave to hold). The Demon will now be yours, but the Mushroom Men will be attackings both you and your demon, and you won't be able to attack them back.
As I said, no idea why you'd want to do this, but still. It's something. :P6):