For lack of a better way to explain it, you appear to be running a less optimal version of the mono-black devotion builds that have been doing quite well since PT Theros. One obvious option would be to look up the decklists on magic.tcgplayer.com and just mirror those, or you can tinker with it and, I think, come up with a slightly different twist on the deck that will be even stronger.
So some specific card choices you've made that I would change:
Lifebane Zombie - He runs MB in the monoblack versions that are seeing play, but to be honest, I would have him in the SB. The deck isn't designed to win via early aggression by any means so the 3 power, intimidate is actually not as relevant as you would think. The ETB ability has some promise, but at the end of the day, they only affect 2 of the colors and it's not a guaran[mtg][/mtg]tee it manages to hit anything (you'll actually whiff quite a bit these days).
Necropolis Regent - This choice seems amusing enough as you could, in theory cheat it in via Whip of Erebos but unless the rest of your stuff is connecting, it's no big deal. It also doesn't leave any residual impact unless everything else hits. I personally ran 2x Abhorrent Overlord as my big bomb creature as you could either hard cast it with Nykthos, Shrine to Nyx and get a ton of tokens OR cheat it with Whip, get a 6/6 flying lifelink AND at least four harpy tokens that will stick around after he's gone. Play him with a few other things in play and you will literally flood the skies with tokens, negating opposing Desecration Demons while giving yourself an army of flying lifelinkers.
Hero's Downfall - This is the bread and butter spell but I like running with 2x unless you're going against a bunch of PWs, at which point you would want 2x more in the SB. But you need to fill out the deck with a LOT more removal - Doom Blade, Ultimate Price, Devour Flesh, and Pharika's Cure are definite MUSTS in some combination across the mainboard and sideboard. Personally, I recommend 4x Devour Flesh in the mainboard as they give you the most options - you can actually target yourself, sack your Gray Merchant of Asphodel, gain four, and then reanimate him for another hit with the Whip - in addition to dealing with threats with hexproof or pro-black (Blood Baron of Vizkopa is a thing I hear). Pharika's Cure helps against aggro and mono-blue, Doom Blade is just a nice tool against any non-black, and Ultimate Price does a good job in the mirror or against another devotion deck.
Rescue from the Underworld - I could see running 2x of this and using a Gray Merchant to go get another Gray Merchant could be fun, but in reality, I wouldn't run any of them in place of running at least 3x Whip of Erebos as it can reanimate multiple targets AND gives lifelink to the whole team.
Corrupt - I get the idea here, but it's really just too slow to rely on as a means of solid removal. Again, maybe 2x could work, but that's probably all.
Strionic Resonator - I don't really see where this is going to do that much for you and a lot of the time could end up as a dead card. I'd drop them as the mechanics at work are solid enough as it is.
Sanguine Bond - I had the same thought when I brewed my version, but at five you really want to be casting Gray Merchant, not an enchantment that won't do much for a while... I see how 4x can easily make a single Gray Merchant a lot bigger, so maybe 2x could work, but it'll probably not make as large of an impact as you might think.
You're spot on with the land, but I had 3x Nykthos, which I felt was enough to ensure you could do some ramping rather consistently, but don't end up with a ton of colorless mana or have trouble hitting your drops.
Cards you're missing:
Underworld Connections - Simply put, one of the biggest weapons mono-black has comes from the sheer card advantage you can generate both with Erebos and Underworld Connections. Sure, it costs you a life to draw a card, but if you draw a Gray Merchant when you didn't have one, well, now you're draining for four and can draw another three cards before you would be back where you started. This lets you rip through the deck at double the normal speed (or faster), ensuring consistency in your land drops and everything else.
Pack Rat - I liked this little fella for a few reasons. Most people just like it for the potential to take over the game with the rats (definitely a real possibility), but I liked the ability to turn dead draws into meaningful plays. Draw a 7th land, just dump it and make a rat token (which has a CMC of 2, including adding one to devotion no less). Have Abhorrent Overlord in hand and a Whip in play? Drop the Overlord during their end phase, enjoy the added rat token, then whip the Overlord, swing with a 6/6 flying, lifelink, haste, gain 6, it exiles at the end of turn, and oh yeah, enjoy those 6 harpy tokens you just created - all for four mana. Really this little fella forces the opponent to have answers while also giving you blockers in a pinch.
Thoughtseize - I really wasn't sold on this card until I was able to use it and... yeah... it's that good. Being able to just see their hand is one thing, but picking the best card and ditching it is an added plus. If not in the mainboard, you need at least 2x in the SB (where I would also add 3x Duress).
Now, these suggestions, shy of Abhorrent Overlord, will bring you a lot closer to the same deck that EVERYONE is playing these days. That's no fun, right? So you can spruce it up a bit in a TON of ways. For instance, perhaps your weakest matchups are against RDW and Esper Rich-kid... err... Superfriends. So what can you do? Well, cards like Profit // Loss in the SB will totally HOSE RDW as a LOT of their aggro guys only have one toughness. They play any combination of Legion Loyalist, Foundry Street Denizen, Firedrinker Satyr, Firefist Striker, Goblin Shortcutter, or Arena Athlete? You're almost guaranteed to at least two-for-one them if not more. Esper giving you fits? It'd be easy enough to run 4x Blood Crypt in the normal manabase to add in things like Dreadbore, Anger of the Gods (although you would want more than four red sources), or even one of the most overlooked but powerful cards in the current format, Slaughter Games. Think about it, you can easily run one Slaughter Games in the MB and take a guess based on what colors they're playing... then you get to look at their hand AND THEIR ENTIRE DECK - likely hitting several cards. If you ever needed some info for how to sideboard, there it is. Nevermind the potential benefit of exiling their most powerful stuff. If you prefer a different swing, run Godless Shrines and Temple of Silence to get access to Obzedat, Blood Baron, or even off the wall stuff like Merciless Eviction. Or, run Watery Grave and Temple of Deceit to splash Ashiok, Nightmare Weaver and Cyclonic Rift.
The point I'm making is, take the main pieces of the current shell as there really is no reason to try to reinvent the wheel, especially if you're trying to make the same thing. Then, make some slight deviations here and there that can let you totally dominate the mirror-match (as you WILL hit it running mono-black) and to help in your weak matchups.
I hope some of this helps. You really do already have the majority of the money cards in the list, so it's not like you're that far off from a finished product that will be really nasty to deal with at FNM. Good luck!
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