Level:40^^^ You must be in The Crypt of T'haen: Endless Twilight, for the quest Crushing Betrayal. Approach him and he will activate after his speech is over. If you don't have enough dps(it is reported that he heals), destroy "a magnificient coffin" at 94,56,-66
CoT:Vengeance (raid), if you don't dps him fast enough, he summons 3 waves of mirror images of himself (adds). This seems to have changed now, more recently it seems he does not summon them for 3 min, then they add at 15 sec intervals. They will appear even after he is dead, so it's a script. IN NON-RAID: Toward the end of Bloodline Chronicles you also have to kill this mob here, the script is different, you must go find the coffin in a room beyond and destroy it first before confronting him, or he will not die, he will heal himself. There's a room full of coffins, one darker one.
