The first thing I look for to improve edh decks is seeing if there are any better versions of cards being used. For example
Cultivate 
instead of untamed wilds. Other examples I found are ur-golems eye could be
Everflowing Chalice 
or
Thran Dynamo 
, sol grail would be better off as
Golgari Keyrune 
or other mana producing artifacts. Ivy element could be
Primordial Hydra 
- one less counter to begin with but doubles every turn.
Another thing I look for are cards that don't seem that great for the deck. Nightmare can be great for edh but with only 10 swamps it probably won't be very impressive for its mana cost. Rites of Reaping doesn't seem like a good card for a 6 mana sorcery. You should be able to find something better without too much trouble. Caustic Tar doesn't seem like the best way to make an opponent lose life. If you want an enchantment that does that you could try
Baneful Omen 
. Makes all opponents lose life instead of one, can do more( or less than 3 damage), doesn't require you to tap a land, and can work with stuff that manipulates the top card of your library (unholy grotto, worldly tutor). Or many other black cards instead. Fevered convulsions doesn't seem that great at killing stuff. Things are usually pretty big in edh and if you're using it to kill your own creatures you're much better off with
Skullclamp 
. Ancient ziggurat should probably be replaced. Not being able to provide mana for non creature spells or abilities is a pretty big drawback and you shouldn't need too much mana fixing in a 2 colored deck.
The third thing I try to do is name some cards that I think would be good for the deck.
Drana, Kalastra Bloodchief 
would be one. It's an edh staple since it can repeatedly kill creatures and attack for a lot. Could also be a good target for Jarad's ability.
Sewer Nemesis 
should be a pretty big creature in your deck.
Harvester of Souls 
- 6/6 deathtouch that draws you cards whenever a nontoken creature dies seems great for this deck.
Grave Pact 
. Your deck is built around sacrificing creatures.
Grim Flowering 
could be an interesting card to try here.
Ghoultree 
- a huge creature that shouldn't cost much mana to cast in your deck. And finally
Lord of Extinction 
which should be the biggest 5 mana creature ever.
You also need to replace neurok stealthsuit since it has a blue activated ability.
Lightning Greaves 
or
Swiftfoot Boots 
do the same thing while giving a creature haste.
