This quest is not available in game.
Kill Glutton, Mordresh Fire Eye, and Tuten'kash.
Although Amnennar is the primary threat here, he is far from the only. You need to slay his lieutenants as well if their hold on the Downs is to be released.
Glutton guards the beginning of the Spiral of Thorns.
Mordesh stands atop The Bone Pile inciting the Scourge into a ravenous frenzy.
Tuten'kash resides underground. In order to bring him out, you must strike the gong in The Caller's Chamber three times.
Good luck, brave hero.