Chaos

First, what are you going for, here? There are a lot of different angles you seem to be running in this deck. I see some targeted corruption (Esl'ian), some direct damage, using some of the disease synergy, and a bunch of Undead.

It seems like the goal might be to use Dark Abyss to throw something valuable (like the Bloodthirster) into the discard pile, then using Raise Dead to get it into play, then maybe the Undead to make the sacrificing a one-way trade? Then, I guess Brutal Offering to provide a reset, if need be? Something like that? If that's the goal, we can totally make this work.

Suggestions
Replace Tzeentch's Firestorm with Plague Bomb.

Drop the Clan Moulder's Elite. They don't really fit the deck. Why not replace them with Sorceror of Tzeentch?

I feel like the disease theme is tacked on here. I may consider dropping it, and getting some better support. But I'd see how I liked things in a game or two before going that route. If you Brutal Offering a Bloodthirster, you aren't keeping any units with Diseases on the board long enough to power your Beasts of Rot (nor are you likely to keep your Beasts of Rot around).

A deck like this could actually use something like Necrodomo's Prophecy (I know, I'm shocked to see it used, also) to help power it.

If you're using Raise Dead for the Bloodthirster (I don't know if you are, but if so), I'd drop the Braying Gor. He doesn't help with your Undead units either, after all. How about adding Warhounds instead?