put duress in the sideboard, in place of the xatrid necromancer, and another pithing needle. read the bones is quite useless in this type of deck.

swap scryland for simple swamp also add at least 1 more nykthos land

Listen to Boza You have no other humans so Xathrid does nothing in this deck Duress and another Pithing Needle would really help the sideboard. In Addition I might mainboard one or two Liliana of the Dark Realms she helps with pack rat.

I run a deck pretty similar to this. Instead of Nightveil Specter I use Lifebane Zombie as a way to get and idea of what the opponent has in store and maybe get rid of a creature. And I run 1 of Sanguine Bond as it has so many great combos with Gray Merchant of Asphodel , Whip of Erebos and Desecration Demon (if it's constantly getting tapped, sac it with Devour Flesh and deal a ton of damage while gaining life)

add 1 Read the Bones in place of one whip. Put the whip in the SB instead of 1 ratchet bomb. You will need it only vs aggro.

I like it! +1

have you thought of Rakdos Shred-Freak in place of a pack rat or two? It's quick simple devotion that outputs damage very quickly.

Davik100 pack rat is way better than freak

Tenacious Dead instead of them Pack Rat is my suggestion

Liliana actually isn't a bad sideboard for MBD now that I think about it. It's your only answer for Thassa, God of the Sea and other Gods outside of Thoughtseize and hoping they don't topdeck one. I'd take out a Pithing Needle and a Ratchet Bomb to squeeze a couple in there.

For Pack Rat to work at his best you need Mutavault out. since you only have 2 Mutavaults, i would either try to get 2 more of them and add a fourth pack rat or just ditch the pack rats all together. Another 2 drop is probably your best bet since you want to Thoughtseize them turn one. i agree with the others you could go with Rakdos Shred-Freak he is a pretty good card, but i like to use Thrill-Kill Assassin i love unleashing him and having that threat of death touch every turn i attack. All of your other stuff will deal with the devotion for Gary. Also i would probably drop one of the shrines to Nykthos, Shrine to Nyx , and a swamp to add in 2x Erebos, God of the Dead . i thought i would need 2 Shrines also, but with all the drawing you only really need 1, and 25 land is a lot for a mono colored deck. Plus Erebos works so good with Devour Flesh . Also i would replace Duress with Dark Betrayal in your sideboard for the mirror match. Here is my deck if you like to just check it out let me know what you think The Dearly Devoted, I love mono black! +1

Strionic Resonator to copy all your ETB abilities.