This quest is a ring event (best take some dps with you) - 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.
Start killing a risen brittlebone skeletons in the inner room. When you kill the first one, then 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.
After you have cleared these, Tralb will "call for reinforcements". This will spawn a lot of an invoked deathwrap in the room. Start killing them.
When you have killed them all you will get a message that says "The deathwraps have been defeated". This will make the Mystic attackable.