This quest is a ring event - head to Ganak's Old Crypt ( 607, -103, 935 ) /waypoint 607, -103, 935 and then make your way to the inner room of the crypt.
See if Tralb the Mystic is up (top level at the rear of crypt) - if not then he is a 45 min respawn, so camp him or come back later. He will be non-attackable at this point (re-spawn is over an hour). Skeletons won't attack you if you have good faction with Synod Reet.
Start killing a risen brittlebone skeletons in the inner room, and on the upper balcony. Once you started to clear the room, assuming Tralb is up, you should get a green message on your screen saying that you have "upset the shaman". Continue killing a risen brittlebone - you will need to kill 20 or so of them (or clear the entire inner room and upper balconies).
After you have cleared these, Tralb will "call for reinforcements". This will spawn a several an invoked deathwrap in the room, 4 on balconies, 4 in the inner room, with about 6 bristlebones. Start killing them.
Several of the brittlebones will undoubtedly respawn, but once the deathwraps spawn, you don't need to kill them again, just the deathwraps. Once again, Skeletons won't attack you if you have good faction with Synod Reet, but they are social and will join in the attack if they are close enough.
Kill the deathwraps from the inner room and upper balconies, you get a message that says "The deathwraps have been defeated". This will make the Tralb the Mystic attackable.
The deathwraps will keep respawning until the ring event ends. You don't have to clear them all - just kill them until you get the message.