Very interesting take on mono black stax, My first recommendation from skimming the list is Bubbling Muck. Because Black high tide exists

Merciless Executioner and Fleshbag Marauder are 3 mana edict effects that wipe everything with endless whispers

Contamination in a creatureless deck? much confusion

Rise of the Dark Realms?

Beseech the Queen?

Spine of Ish Sah?

Mimic Vat?

There has never been a better deck for Spreading Plague

Nettlevine Blight?

Dark Petition and Diabolic Revelation could be good tutors

What a fun deck! Some cards that popped into my head when I looked through it (in no particular order): Nephalia Academy, Font of Mythos, Horn of Greed, Plague Wind, Barter in Blood, Necromantic Summons, Necrotic Plague, Torment of Hailfire, Torment of Scarabs, Yahenni's Expertise, Corrupt, Nightmare Incursion, and Pestilence.
I think the Font or the Horn could work especially well. Everything else is just something to consider if you find a need for more of a certain mechanic, since they're all covered by other cards in your deck. Sorry if you've already debated some of these. Great work!

Love those thoughts discipleofgary! And LeoSushi, dannng nettlevine blight! Better hope you have some enchantment destruction!!

This is such a fun and brutal deck, I wish I could +1 it twice.

Dooope. Fantastic design space and utilization of a weird commander.

Dust Bowl is always good alongside a stack of basic lands.

Imp's Mischief blue redirection in black.