I really like it. I have something very similar to this deck, almost quite the same creature base. Check out Gray Merchant of Piss on your Grave. Because you're running Gary, I'd recommend Vendetta for even faster turn around on threats. If you feel like being daring, Sorin Markov is pretty fun to use.

Victim of Night hits most things. maybe a bob or two but your curve might result in you lava axeing yourself to death.

I'd maybe main board the Geth's Verdict

Other than that I quite like the deck and idea of it. :)

I think mainboarded Geth's Verdict is a mistake most mono-black players makes, you have more than enough sacrifice already if you ask me.

Yeah, sorry I didn't see the gatekeepers :P

Geth;s verdict is sideboarded due to the above reason, Obliterartor and the gatekeepers take care of that mess

Hey, this is awesome, good work! I run a very similar deck (just with more discard vs creature destruction; I end up playing against a lot of creatureless control).

3 things to mention:

Play around with Vampire Nighthawk in place of Geralf's Messenger. While Messenger is more aggressive and harder to remove, I find that this deck plays better when trying to stall the game out so that you can reach 5 mana. Modern is a turn-4 format after all. In that respect, Nighthawk is a better blocker, more evasive, gains you life (especially to make up for the Thoughtseize and Phyrexian Arena life loss), and is one of the few answers to Emrakul, the Aeons Torn black has in Modern. I'm not necessarily suggesting a change, just saying play around with each. I ran Geralf's for a long time before deciding that Nighthawk played a more versatile role for me. Devotion won't matter if you can't survive past turn 4!

In terms of removal, Go for the Throat is dead against two of the most popular modern decks, Affinity and Tron. Victim of Night is generally the best 2-mana black removal against tier-1 decks, but I personally run 1 of each in my main, along with 3 Dismember and 2 Disfigure to keep my removal package versatile yet efficient.

Finally, the only suggestion I think is actually a necessity. Swap out a Nykthos, Shrine to Nyx for an Urborg, Tomb of Yawgmoth, and maybe even a swamp for another. 4 Nykthos really lowers your consistency when you're playing cards like Geralf's Messenger and Phyrexian Obliterator, and an Urborg or two will help a lot by letting them tap for black.

Oh, and Slaughter Pact in the board for Splinter Twin decks and Pod decks trying to infinite combo!

hey man thanks for the advice i'll certainly look into it.

If you're still looking for suggustions, Malicious Affliction could help conserve mana each turn for more spells.

I would put Hero's Downfall in the sideboard, and replace it with Go for the Throat. It hits most targets, and when it doesn't, you side it out.

Heros downfall is there for the destroy target planeswalker thing

Not very many decks run Planeswalkers, and you can side in downfall for when they do.