Random invulnerability on ghosts.

Many times killing ghosts in Tomb of Primeival Kings the ghosts will randomly run off gaining invulnerability and keeping you locked in combat. It wouldn't be so bad except they can run for long distances and until they reach a certain spot they are invulnerable to damage.

This problem exist for all PoF mobs to some degree. Even the awaken that were added to core maps for the invasion events. They become invulnerable even while standing next to you.