Darnavan

Darnavan was sent by the Argent Crusade to infiltrate the Cult of the Damned, but fell to Lady Deathwhisper's brainwashing. Now a true cultist, he'll attack you on the first mob wave during the Lady Deathwhisper encounter, but needs to stay alive for. After killing Deathwhisper he will turn.

Strategy
Darnavan uses different warrior abilities, but doesn't hit that hard. He needs to be off-tanked for the whole fight.