Slay Grol the Destroyer, Lady Sevine, and Archmage Allistarj. From their corpses take the Amulet of Grol, the Amulet of Sevine, and the Amulet of Allistarj. Remember to USE your Enchanted Azsharite Fel Weaponry to weaken the triad. They cannot be killed otherwise - but use them wisely...
To reach the Defiler, you must first destroy the triad of power and recover the amulet pieces. When you recover all three pieces, return to me and I shall give you further instruction.
Go [player]! To the Blasted Lands! Sevine, Allistarj, and Grol must fall!