just a suggestion, Desecration Demon works really well with jarad and varolz

I really think this deck has pretty good synergy. It is a little slow but there are good cards for GB that can slow down the first 2-3 turns to setup your graveyard work. See below.

For now, Abrupt Decay x4 will help in the early game to slow the opponent down and in the later game to handle things like Domri Rade , Loxodon Smiter , etc.

Doom Blade is a good mainboard card. If you think you're getting to many spells you can always do two Abrupt Decay and two Doom Blade and sideboard as needed.

With so much graveyard work going on, you could also do Scavenging Ooze . That is pretty strong too.

Other cards that just pop into my head that may or may not be of any use:

Jarad's Orders - to help the graveyard work.

Dreg Mangler - Why on earth isn't he in there?!?

Putrefy may be a good sideboard

Rancor is a great card advantage, until it goes out.

Vraska the Unseen - Seems like it could have good possibility if you already have one. I would trade for her though.

Golgari Charm - This is a great card, especially for early game against 1/1 or 2/1 creatures, which are popular now.

Grim Return - Actually may be great in this deck.....

Grisly Salvage - great card to get things going in the graveyard.

Elixir of Immortality - may get some cards back through your library and give your life a quick boost. No more than 2 though.

Treasured Find - Perhaps to pull something back out you want...maybe

Deadbridge Chant - A bit of a later game card, but it would be a bomb in this deck!

Kalonian Tusker - GREAT ratio card. 3/3 for two ain't bad, son!

Witchstalker - a MUST have card now days for BG decks.

Lifebane Zombie - another great sideboard card

Rot Farm Skeleton - not entirely sure why, but this looks like it has great synergy.

I know this is quite a few cards and most may not really have a spot, but it is what I could think of to offer ideas. When Vampire Nighthawk rotates out, it will be a sad day. One of the best cards that no one really plays....

-2 Disciple of Bolas +2 Kalonian Hydra ?

dude9914, that is the perfect recommendation. I don't know how I am going to find it, but that's PERFECT. That card will wreck the world! I am off to find a pair!