It you're playing casually Damnation would be better than Black Sun Zenith also.
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I like the idea for your deck. I put a deck together that's similar but I thought it would give you some ideas. http://www.mtgvault.com/ViewDeck.aspx?DeckID=164396 I put Bloodghast and Kalastria Highborn in so you can make dying Vampires worth counters on Bloodchief Ascension or you can sack Bloodghast to do damage and bring him back with landfall and do it again. That would also be awesome with Blade of the Bloodchief and Bloodthrone Vampire so if you didn't pull what you needed for the combo you would stand a pretty good chance of beating your opponent to death brute force.
Thanks for the feedback. I'll have to put this deck back together play with your suggestions.
I thought you might consider the Primal Below and then Overwhelming Stampede combo which I have found can be pretty devastating. Even though Garruk doesn't fit the theme he would definitely add some stomp to this deck. I'm not a big fan of genesis wave but I see how you could make it work in this deck. Overwhelming stampede is definitely your kill card.
I made a deck with a similar idea a while ago. I never got around to putting the finishing touches on it but it was a pretty successful deck. Take a look and see if you can't pull some ideas for your deck. http://www.mtgvault.com/ViewDeck.aspx?DeckID=113892