Speak with Layo Starstrike near the Valor's Rest graveyard of Silithus, showing him Rabine's Letter.
West of Un'Goro Crater, via the northwestern ridge, is a harsh wasteland - Silithus. Few know that an ancient war was waged there between the night elves and a malign, alien power. The evil was sealed, but now we suspect that forces work to stir this... unspeakable horror.
Seek out Layo Starstrike, one of our agents in Silithus; look for him at Valor's Rest, the final resting spot for the fallen of the wastes. Give him this letter, and he will give you his trust accordingly.