…steal Harpy eggs.
While I have arrived at the point where only 1 thief was alive, the quest never finished. Me and maybe 5 other players kept killing harpies, resurrect Skritt Foragers for like 15 minutes but the quest never progressed, there was still just 1 thief left (we never saw him I think).
Happened on Gandara, 22/09/2012 at 22:00BST.