I died during my first attempt at the Jar of Souls event, without killing all the monsters or surviving the countdown (obviously). Then I resurrected at the nearest checkpoint and went back there, thinking I'd restart it. When I clicked on the JoS, however, I got a burst of treasure and a notice saying I'd completed it. When I go to look at my achievements, there is one there for the Jar of Souls. This was during solo play.