I think you are probably too heavy on creatures. Since you often win just by playing Gary, you don't really need to be super aggro as long as you're controlling the board in the meantime.

1-for-1 removal spells and things like Inquisition of Kozilek and Thoughtseize could probably replace up to half of your creature slots. Also, while the Planeswalkers you have here are both good, they're a little mana heavy for modern -- really, Liliana of the Veil is the only 'walker I can see maindecking.

The Dragon's Claw in the board isn't that great for you, I don't think. Really, the way you want to be stopping burn in my opinion is with effective creature removal and Duress effects. Also, Helldozer is a little bit slow.

I think MOST of the stuff in your maybeboard is dismissable -- but there are a few really great cards there for you. In particular, I could see 1-2 Kalitas in the main. Same thing with Gatekeeper. Fulminator Mage can be REALLY good here, too (over Helldozer).

You may also want to run some number (maybe 2?) Urborg, Tomb of Yawgmoth. That lets you tap your Nykthos for black in a pinch, and perhaps allows you to run some utility lands, such as maybe 1-2 Mutavaults or something.

Last thing... I think you go all-in on Obliterator, at 4x.

Anyway... that's all I got at the moment, this is just all that sprung to mind when I saw the list. I actually played Mono-Black Devotion in Standard for over a year, so while the format isn't the same by ANY means, I feel like I have a good idea what the deck is "trying to do".

Oh! Also, and this is much more relevant if you go all-in for Phyrexian Obliterator...

but a potential source of MEGA VALUE for you (especially with Urborg, TOY in the list) is the underappreciated Timeshifted land, Arena.

Thanks tettsui77 ! I appreciate the feedback... I'm actually really trying to limit any non-permanent cards in here so that I can feed gray merchant, reiver demon, sorin etc... Definitely love the urborg and arena recs, though... going to toss them in here.

tettsui77 ended up using about 90% of your recommendations after play testing. Great suggestions all around, much appreciated. The hand disruption/removal is key here since i don't have a ton of powerful early plays. Wish I had damnations and lilianas to throw in but that'll have to be added as time goes on. Arena was a bit slow so i pulled it for now and i'm sideboarding k. command for artifact protection so that's why i've got some dual lands in there. lmk what you think of the current build!