Make sure the to tank this mob back to the wall in one of the far corners of the room. Benach will kick the group from time to time and since and may reset if you get kicked to far.
Benach will talk from time to time and will say something about poison, cold, etc. When he does that, he will cast a very nasty DOT on the tank. Make sure to cure it as soon as possible.
He will from time to time call forth a large mob of a spectral tome so be prepared to AOE them. These books popped twice for me and totaled to about 1,300,000 HP.