Right-click on the stone in the center of the four Elders in the Sanctuary of Tears ( -7, -632, -728 ) after completing the quest Gathering of the Elders. You will be zoned into The Ultimate Sacrifice and this quest will automatically be offered.
Witness the ritual. This will take several minutes but no fighting in involved.
Right-click the stone in the center of the ritual area to return to Withered Lands.
Speak to Silnas at -52, -626, -617 to complete the quest.