It's this thief who was left over after the fight where the scoundrel is introduced in Nightmare Act 2.
He was kinda left over, kept attacking but he's unattackable. He doesn't do any damage, but I'm not able to do any damage to him either.
When it happened at first I just shrugged, and just tried to keep up with my buddies and when we went into the area with the two dungeons with the orbs he dissappeared. To my surprise however, when we came back to put the orbs on he was still there and started attacking again.
Here's a video of that moment: