Does ANYONE know how to get this item?

Unfortunately Tome of Obtenebration does not exist on Single-Player only Multiplayer.

BUT there is a way you can get it, after my fifth play through I always added the item to my inventory and used it on Willhem.

You have to enable console then write:
addthing tomeofobtenebration
and then press enter
and you will have it summoned on the ground. pick it up and use it.

But I have to warn you, Obtenebration is flawed, you can only use 1 level. I suggest you to use a mod for it to expand it. there is a mod called "Within the Darkness" that has alot of new disciplines, that can be fun but I suggest that only after your first play through is done.

--PS-- you should have set the thread as "Question" -- it would be more fun answering it.

Sorry didn't think to set that. And Thanks for the answer :D

So there are more Disciplines?

There is a mod called "Within The Darkness" that adds almost ALL the disciplines, and reconfigure the ones already in, but I dont suggest to use that until you finished the game at least once or twice. and the game is varied enough for you to play at least 7 times. and btw it has 3 possible endings.

Other disciplines NOT found in single player are not many, in fact I can remember these are all of em:

Animalism
Auspex
Blood Magic
Blood Rituals
Common
Dementation
Dominate
Faith -- Human Only
Hands of Destruction
Lure of Flames
Mortis
Numina -- Human Only
Obfuscate
Obtenebration
Physical
Presence
Protean
Serpentis

The one that you wont find is "Dementation", you can summon it by its name:
addthing tomeofdementation

btw. if you like press a +1 on the post, the list was not easy to come by (^_^) and in Alphabetic order even (-_-)

thanks anyway.
If you have more questions, please, let me know.

Thanks

anytime.