Spiteful Apparition can be found in the Forge of Souls of the Frozen Halls in Icecrown, they appear out of thin air and attack a random party member, then disappear. It is very difficult to kill them before they use their attack and despawn.
Abilities
[Spite]ωϖ 15 yd range—Deals 4500 to 5500 Shadow damage to enemies within 15 yards. 3 sec cast. 7200-8800 on Heroic