Speak to Boulden Smitestone and pick up the quest, Protect the Priest's Ship!. Make sure to continue through all the quest dialogue, not just accepting the quest as soon as it pops up, in order to start the event. Slay the skeletal orcs as they spawn and eventually a Rallosian Captain will spawn. Kill him and continue slaying the skeletal orcs until no more spawn. This means that Boulden has finished his warding spell and it safe to leave the instance. Speak to him to do so.