Love the deck list, +1 from me! I've got a few suggestions none the less, so feel free to consider/disregard/whatever. I appreciate some of these are pricey in places and perhaps a little too good-stuffy for what you might be after, but I'll list them anyway.
Black Sun's Zenith . It's just too useful to not use. A reusable board wipe that gets around indestructible? Feels like a missing piece, especially considering the mana you have access to in mono-Black.
Praetor's Grasp ~ A bit of a meta call usually, it feels right at home with the deck you've created. Extracting something important from someone's deck in itself is always a good call, but being able to play it yourself? There aren't many things more satisfying than stealing Curiosity out of a Niv-Mizzet deck, but casting somebody else's Kozilek is one of those things, in my opinion. Sometimes this is just a poor Sadistic Sacrament, which you already run, but other times it's just wonderful.
Exquisite Blood ~ I appreciate that this omission may well be on purpose to not run an infinite combo, but it's one of those cards that just works so well in multi-player EDH, not least because it allows you to gain life that no one else can catch up to if Erebos is on board. It does paint a target on your head though, so I can see reasons to leave this card out, but I figured I'd mention it.
Tragic Slip ~ Stuff is always dying, and although spot removal isn't as necessary in EDH as in some formats, being able to drop Eldrazi and Avacyn for just one black mana is magnificent. It's conditional removal, but cost efficient and deals with keywords that are unsightly and all too common.
Mindcrank ~ Another infinite combo piece with Bloodchief Ascension, thus painting a target on your head, but winning is winning. You make people lose plenty of life, so this is usually pretty good. Fuels Geth too. It's another meta call though, as there may well be other graveyard abusers in your meta; the Lazav player at the table for one would love you forever if you make full use of this card.
Skithiryx, the Blight Dragon ~ The quintessential 'fuck you, mono black player' card. Infect is one of those keywords not every group appreciates, but sometimes it's the only way to win out of nowhere. Another card I completely understand people don't run out of principle, but still worth me mentioning in case it slipped under your radar.
Rune-Scarred Demon ~ Abuse-worthy tutor on a decent creature body. Not to everyone's liking because of the mana cost, but you run Whip and Mimic Vat, so this has just too much potential.
Mikaeus, the Unhallowed ~ You're running black. This card needs no real introduction from me, but recursion on certain creatures that you're running, either with ETB or death triggers is enough to merit this, even if you don't want to use it as a tool to go infinite with.
Bloodgift Demon ~ Phyrexian Arena on a creature, so a good target for recursion once again. Considering you already run Arena and Necropotence, this is probably sub-optimal, but might be worth a spot.
Temporal Extortion ~ Giving opponents choices is nearly always a bad thing, but I have a soft spot for this card and you sound like someone who may well appreciate the beauty of this card.
Beacon of Unrest ~ Whether it's your graveyard or another, this card is an absolute house. From my experience though, it's hard to get hold of.
Once again, love your list; these suggestions are just cards I've found to work very well and ones that you could perhaps consider. =]
I've got a stack of about 150 cards that didn't make the cut and all of these are in that pile, so I did consider them. Maybe in a different meta I would play them but currently this list works perfect for me :P I'm also in the process of getting a Damnation as well and I've tried to stray away from infinite combos. Exponentially large combos are okay but I prefer to keep Mind Grind out unless I'm in a meta that is more competitive.
How much work does obliterator do? I have one in my monoB but i feel like i should just put it back in my trade binder. All it ever does is turn on devotion for Erebos so he can get turned into a bug or tucked ....
Obliterator is more of a rattlesnake than anything but if it's safe I will let him loose on the playground for some damage/havok.
The thing about Obliterator is that no one wants to interact with it, and you should be using that to your advantage. Someone could be plotting to hit you with a massive loaded up voltron but as soon as you play Obliterator, the whole game changes. At worst, he offers devotion and helps buy time to set up another threat. At best he completely wrecks someone's board if they get too greedy/reckless.