Destroy Razelikh the Defiler and return the Severed Horn of the Defiler to the Fallen Hero of the Horde. You will also need to return the Ward of the Defiler so that the Fallen Hero can destroy it, preventing it from ever falling into the wrong hands.
Razelikh's lair is atop the highest mountain in the Blasted Lands. The only way to the lair is through the use of the teleport runes near the mountain. As long as you hold the amulet, you can freely transport up to the lair. Once you reach the lair, you must conjure the beast. The amulet is your key, <name>. Use it on the Altar of the Defiler.
Should you defeat the Defiler, bring back his horn. It will become a symbol of hope for all that view it - proof that heroes do exist in this war torn world.