Enter Greenscale's Blight and slay the corrupted Lord Greenscale. This is a 20 man raid dungeon and will take great amounts of teamwork and organization to even get to Greenscale. When you do get to him he has 4.2 million health and a four phase complex encounter. After slaying Greenscale collect the Death Corruption to complete this epic quest.
To see the whole quest series at a glance, see The Saga of the Endless Guide .