I second
Plague of Vermin as well as lowering your curve. I'm not sure what your meta is light, but your deck seems like it would move along pretty slowly with so many high mana value (CMC) cards. I took my deck into a lower curve aristocrats creature recursion vein and using sacrifice with drain & gain as my primary wincon. Some cards I use are:
Bloodghast,
Nether Traitor,
Bloodsoaked Champion,
Oathsworn Vampire,
Gravecrawler,
Reassembling Skeleton,
Mikaeus, The Unhallowed,
Bastion of Remembrance,
Vito, Thorn of the Dusk Rose, and
Marauding Blightpriest
I'm still creating as many black creature tokens as I can too. I have a few of the same as you. I'm not running
Chittering Witch as I unfortunately get to play 3-player once in a great while and mostly just 1 vs 1, so take that into consideration too. I also used to run
Dreadhorde Invasion, but it always felt so-so and a bit slow to me. You might like
Sengir Autocrat. I recently added in
Blight Mound to test, but I haven't had a chance yet.
One card that's brutal for your opponents is
Contamination, but it doesn't quite fit the theme. I decided to take it out as it was pretty mean. If I ever play in a higher-powered 4-player meta I may put it back in.
I like the idea of
Carrion and
Thornbite Staff that you're running. I may borrow those.
You really might want to add more draw to your deck.
Necropotence is $$$, but if you can get a copy it's worth it.
Plumb the Forbidden has been pretty good for me as well.