If you need another Crest of D'Morte go into the D'Morte Burial Chambers again and touch T'Haen's Coffin at ( -52, -11, 45 ) Copy/waypoint -52.06, -11.12, 44.55
Kill Izinar Malkevantrii ( -5, -4, 74 ) Copy/waypoint -5, -4, 74 and his two assistant conjurers. You will receive a new Crest of D'Morte, and your old one will be destroyed.
Note: He will attack even if he's a grey con to you.
Find and destroy a magnificent coffin at ( 94, 56, -66 ) Copy/waypoint 94, 56, -66 from which it is said that T'Rath the Usurper receives his power.
This step is optionial, however if you don't destroy the coffin he starts healing at 35%.
To reach the coffin go to the narrow bridge at ( 0, 57, -59 ) Copy/waypoint 0, 57, -59 and cross the chasm. Take a left then follow the path to the room with dozens of coffins.
Destroy T'Rath the Usurper atop the central pillar at ( -7, 61, -26 ) Copy/waypoint -7, 61, -26. You will receive a new Crest of D'Morte, and your old one will be destroyed.
Note: This last step, is identical to the quest A Vicious Twist (Hand of Marr series) or Crushing Betrayal (Brethren of Night series). Consider doing at the same time.