This quest is still very bugged, but I got it done today. I thought I needed "Undead Gypsies" even though the quest says "Undead Brigands". But as it turns out, the game didn't want either those, it actually wanted "Undead Gravediggers"! Got that straight? Uh, me neither.
But anyway I was able to complete the quest today by defeating a bunch of Gravediggers. Weird.