In the ruins, players might encounter an angel fighting back the demonic horde. He will ask for assistance in stopping the demon lord Tarthess, who is trying to resurrect its ancestor and master buried nearby. The monster is found in the end of the bridge, past several Barbed Lurkers and Warscarred Marauders. Killing it will finish the event, and the angel will thank The Nephalem for help.
There is a chance to find a Resplendent Chest nearby, but it is independent from the event.