Enter the "Tomb of Night" instance. This will zone you and your party into "Tomb of Night: Retribution" (Bloodline Chronicles Adventure Pack is required for this instance)
Spawn Dvith N`Zur ( 57, -41, 114 ) /waypoint 56.5, -41.38, 114.25 by killing the five groups of vampires around the base of the waterfall (big cave level 3). When he spawns, he will not be attackable.
Kill the Nightmares of Dvith to make Dvith N`Zur respawn as an attackable foe. He may respawn underwater at the bottom of the waterfall.