This quest is part 5 of an 11-quest series in the Bloodline Chronicles Timeline. Make sure you enter the "normal" Tombs of Night instance; if you re-enter the Tombs of Night: Retribution instance, the vault door to the Crypt of T'Haen will still be locked.
In the Tombs of Night Instance, return to the Crypt of T'Haen (Endless Twilight) entrance where you saw the spirit of Dryxl T'Rath go when you killed him in the previous quest. Note: Entering the crypt will advance the D'Morte Family Crest quest which requires you find a way into the higher caves.
Resting Chamber ( 90, 57, -88 ) /waypoint 90, 57, -88. The Resting Chamber is actually around/behind T'Rath, across the narrow stalactite bridge (the same location as his coffin, which is helpful to destroy if you want to kill him).