I know these things, they're worse than orcs, they're the reason the orcs went all rampagy. They're called the Burning Legion and their goal is nothing less than the annihilation of all life in the universe.
They can be beat individually and as a group, but god help us if Kil'jaeden or another general arrives. I just... I just dreamed that a few days ago. It took three entire armies holding back the onslaught of demons and risen dead and an entire species giving up their immortality to stop him.