Speak to Slaughtergrott the Vigilant 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 Slaughtergrott has finished her warding spell and it safe to leave the instance. Speak to her to do so.
The information below has been verified with in-game data and the information on this page is correct, even if it looks stupid or makes no sense.Fact: Zonename is indeed "the LMS Intruder", see talk page