Oni are level 28 assassin class mobs that are invisible, but jump out and attack your party if you get close. They're not a big deal if you're ready for them, but if you get them as adds on a harder pull, they can be trouble. Group blocking skills will greatly reduce their danger. Oh wait, wrong game.
Oni are demons of Japanese folklore, since MooShu is a mostly Japanese-themed world. You can read more on the Wikipedia article on them.