Kill Ulag the Cleaver, then speak with Deathguard Dillinger.
When the Scourge swept through Tirisfal Glades, we captured one to study. He had a free mind, and called himself Ulag the Cleaver. But, although his will was free, he was still enthralled by the Lich King.
We hoped to find a way to break the king's control over Ulag, but we failed. Now, finally, it is time to free him.
Free him, and kill him.
Activate the skull stone trigger near the mausoleum to the north. This will lift the magical seal that binds him.
Then... be ready, [player].