I came upon a room with two shankles moving left and right between some explosive blocks. The party mostly ignored them, save for me taking a few pot shots at them, and went on to do a danger room. When we returned (the elevator was back the other way), one of the shankles had stopped moving, while the other was continuing to zip back and forth. Killed one, but the other one just sat there in its invulnerable state.

Did an F2 report on it.