Find and destroy A Magnificent Coffin from which it is said that Dryxl T'Rath receives his power. It is at ( 94, 56, -66 ) /waypoint 94, 56, -66 off one of the small ramps from the platform where you find T'Rath the Usurper.
Destroying the coffin is optional but if you don't destroy the coffin he starts healing at 35%.
To reach the coffin go to the narrow bridge at loc (0, 57, -59) and cross the chasm. Take a left then follow the path to the room with dozens of coffins.