The way to see this through is to stop all damage when you see him turn black for a few seconds... this is your warning.
Stop all DPS for roughly 15 seconds and resume. There will be two buffs that you will see on him about shaded devourers. It looks like a SK ability (Grave Sacrament?? ..need confirmation).
You want someone to be able to see when this buff disappears from his detriments. It is roughly 15 seconds but you may need to see the buff to make sure.
Stop attacking properly and this mob will drop rather quickly.
Good luck.
1. Attack him hard till he goes black/shadowy which is your warning to stop ALL attacks
2. 5 Seconds after he pops up a BLUE ying/yang buff on him
3. If anyone attacks they get an add that will kill them, just let it kill them if they did, add disappears after killing them
4. Blue Ying/Yang Symbol disappears back in till he goes shadowy, rinse and repeat
Note that he does port around the room and tries to kill people, keep group wards/rhs up and you're good.
Extra note: I sometimes get killed because of the stun. He will stun you and then go shadowy which stops you from being able to turn off auto-attack. You get unstunned and autoattack him by mistake. Watch for this tanks, dead easy fight otherwise.
The buff he puts up is the same icon as Grave Sacrament. If something does hit him and adds are spawned there will be another buff icon that has the same graphic. Adds will despawn so just have to tank them till they poof.