Lim-Dul the Necromancer could work, kill your opponents creatures turn them into zombies.

Sudden Spoiling goes great with Pestilence Demon

Damnation All around great card in mono black

Attrition Works well when you have a vast army of zombies

I would definitely suggest Cabal Coffers! You play a lot of Swamps and even Urborg, Tomb of Yawgmoth. You can generate a crap ton of mana with that card.

I can only second Cabal Coffers and Urborg, Tomb of Yawgmoth with your curve. I would even throw in an Expedition Map and a Vesuva for higher consistency and gain respectively. And while we are on land add a Leechridden Swamp.

I would replace the Charcoal Diamond and Worn Powerstone with a more powerfull rock e.g., Thran Dynamo or Caged Sun. Liliana of the Dark Realms will also help your lands.

Karn Liberated, Argentum Armor, Deathbringer Regent, Decree of Pain, Silence the Believers and Steel Hellkite would strengthen your removal.

Sorin Markov is pretty good in mono black edh. At least consider him :)

I am missing Necropotence, Phyrexian Arena, Solemn Simulacrum and a set of tutors in your deck (Demonic Tutor, Vampiric Tutor, and Increasing Ambition).

Consider replacing Pitiless Horde with Graveborn Muse.

Disciple of Bolas would be a good exchange for say Grim Haruspex.

Cheers

I would cut Urborg, Tomb of Yawgmoth. You are running a mono black deck so you dont really need the fixing.

I would play it! It's not in there for fixing. It helps Nightmare, Cabal Coffers and Corrupt. I don't like Greed too much. As mentioned before Phyrexian Arena is far better in my opinion. And even Underworld Connections could be a choice.

Hail Satan.

Nightmare Lash over Strata Scythe. Strata doesn't get a boost from Urborg, Tomb of Yawgmoth the same way the lashes do. Same name printed on the card vs. Swamp sub-type.

Phyrexian Arena over or next to Underworld Connections. Phthisis seems too cute but is probably a lot of fun with a big pay off.

Fleshwrither is one of my favorite sleeper cards in a deck like this. You already have a variety of 4-drops in your deck to grab. Erebos, Crypt Ghast, Percy and Disciple of Grizzly are always great tutor targets. This would also open up other cool 4 drops like Graveborn Muse, Nekrataal and Phyrexian Obliterator. Even goofy stuff like Sangromancer and Squelching Leeches.

Ashes to Ashes

where is you Solemn Simulacrum???

Geralf's Messenger?

Undying Evil??

any creature with undying would do great + Altar's Reap, Culling the Weak the obvious Dark Ritual

Maybe make it more zombie tribal since your commander aims to create does.

Solid deck. I run Gisa as my primary commander and she is a force to be reckoned with! My playgroup identified this early on and now they've tried to put a muzzle on her. Hopefully you don't play with as many blue and white players.

Alright, on to my suggestions:Mimic Vat is probably one of my favorite cards. Target Grave titan, get 10 zombies and a swing from a 6/6 with deathtouch. Target Rune-Scarred Demon and pull 6 zombies and get a filter for your deck. Gray Merchant and drain everybody. All for 4-5 mana a turn! I think you get the picture.

Coat of Arms is also great. You'd probably need to run more of a tribal theme to really benefit though.

Mikaeus, the Unhallowed. Undying demons? Sure, why not.

Grave Pact is better than Dictate of Erebos in some situations.

I also agree with the above suggestion for Phyrexian Arena.

If you care to take a look at the deck I mentioned earlier to get some ideas, feel free: Gisa, My Sinister Sacrificial Sister

Solemn Simulacrum. Since not many people play discard this is a great way to ensure you get 5 mana for your commander and draws you a card.

Thousand-Year Elixir

Demon of Death's Gate

Risen Executioner, Death Baron and Lim-Dul the Necromancer for zombies (although you recently removed some of those cards, so i guess that means you like the demons more then the zombies your general makes) .. if that is the case then maybe Geth, Lord of the Vault or Lim-Dul the Necromancer could fit on theme as generals.

Crux of Fate solid death at 5cmc

Archfiend of Depravity and maybe Stronghold Overseer (both can restrict enemy field presence)

Thornbite Staff to repeatedly use Gisa as many times as you can afford.

Which could be infinitely many times if you also run Phyrexian Altar. Plus Sac Outlets are generally very good to keep around.

Living Death is also good to have if you get late game with a full yard.

If you're sacrificing a lot of stuff, I like to have Blood Artist around for trimming down opponents.