The Nephalem may encounter two Angelic Warriors in the field, next to a damaged ballista. They will say that Imperius sent them to provide a distraction, in order to allow the Nephalem to get closer to Pandemonium Fortress undetected. To do so, they need someone to protect them while they repair the siege engine.
Upon accepting the quest, increasing numbers of Winged Talus will start to pour from the surrounding chasms, coming in waves. Their flow is truly massive, up to hundred at once, so a hero without steady output or sufficient protection may have difficulties dispatching them. Angels' progress in repairs will be shown as a red progress bar.
The challenge is not time-based, but rather requires the angels to perform their repairs uninterrupted, making them stop if attacked. They cannot be killed, and will yell to the Nephalem to protect them from blows.
Eventually, the bar is filled, and Sharpclaw and possibly several Barbed Lurkers will emerge to join the battle. The angels will signal to kill the creature, as its death effectively ends the event, stopping the flow of Talus.