Long ago, the Clerics of Karabor were the priests that resided within the Temple of Karabor, the private sanctuary of the Prophet Velen.
When the Temple was attacked by the Horde, many draenei clerics were taken prisoner and marched outside the Temple ramparts. They were told that they were being taken to Hellfire Citadel far to the north. When they reached the outside of the walls, Teron Gorefiend — long before he became the first death knight — cold-bloodedly murdered every last one of them. Their vengeful spirits remain trapped outside the Temple of Karabor, now known as the Black Temple, to this day. They hold Gorefiend's Cloak and must be defeated in order to obtain this artifact. You must be wearing Spectrecles to be able to see them.