Speak with Molthor in Stranglethorn.
[player], our worst fears have come. Legend speaks of the Soulflayer ruling the ancient troll world through evil and terror, and his power was absolute. If he cannot be stopped now, then his new reign may spread across all of Azeroth!
There is a contingent of Zandalarian trolls who may help us stem this tide of darkness. They have camped on Yojamba Isle, off the coast of Stranglethorn, west of the ruins of Zul'Kunda. Speak with their leader, Molthor, and pray he knows how to defeat Hakkar.