MTG Salvation

Mono black drain. Whittle life totals away, hopefully not too much of our own, and grind out a win!

How about the new Torment of Hailfire?

Torment of Hailfire def has a home here. Probably take out No Mercy even though it's a pet card of mine. A good deterrent but the face can only take so much!

+rep for using Triskaidekaphobia

Have you considered the direct drains like Drain Life, Consume Spirit, Maga, Traitor to Mortals, and Profane Command? They don't seem very good, but at X = 15+ they might as well say "Target player loses the game. Oh, also, you gain a lot of life." Sure, they're just begging to be countered, stolen, or redirected, but I play a decent amount of discard in my deck to get rid of problematic cards (I'm considering adding Thoughtseize) so with smart play they have a tendency to go off regardless.

I never thought I'd say this, but I think you're running too much card draw. In other mono-black lists I wouldn't hesitate to suggest playing Ambition's Cost/Ancient Craving and even Promise of Power but here they're a little overkill since your general draws cards. Sure, not quite as efficiently as the previously mentioned cards, but taking out superfluous draw allows us to play more tutors/disruption/win conditions.

Also, personally, I'm not a fan of the little pingers like Falkenrath Noble and Zulaport Cutthroat. It adds up in a technical sense, and I have killed people with Crypt Ghast Extort, but meh. I also don't see cards like Howling Banshee or Soulcage Fiend doing work at all. I play a creature-light deck because my list is set up to heavily punish playing creatures, but for your relatively light sweeper list I can see playing stuff like Pontiff of Blight and Psychosis Crawler.

I dislike Jet Medallion because it can't be used to activate abilities, and I hate Unstable Obelisk. Yes, it's a mana rock that blows stuff up, but ten mana is steep, even for big mana mono-black.

EDIT: Speaking of Crypt Ghast, where are the mana doublers like Magus of the Coffers and Nirkana Revenant? They're stupid good and allow you to turn massive amounts of mana into massive card draw which translates into more murder.

Also in terms of mana doublers, I think Caged Sun has a home here.

Updating list. Very Good suggestions, the deck has been together since Theros and I had swapped cards here and there but haven't looked at it layed out like this since I sleeved it up.
No Mercy for Torment of Hailfire Fits perfect here and I won't miss the enchantment.
Falkenrath Noble for Crypt Ghast I can lose the vampire because it only hits one player anyways.
Ambition's Cost for Nirkana Revenant I have loads of draw already and its CMC is the same as Erebos.
Ancient Craving for Pontiff of BLight See above.
Jet Medallion for Caged Sun Weaker of the mana rocks, big upgrade.
Mind Stone for Black Sun's Zenith Weaker of the mana rocks, I felt better about the deck with more board wipes.
Charcoal Diamond for Damnation See above.
Coldsteel Heart for Toxic Deluge See above.
I cannot seem to let go of Unstable Obelisk. I've lost too many games because of problematic artifacts or enchantments and mana usually is not an issue.

Lovely! I contend that Erebos is the best option for mono-black goodstuff. I've been toying with him since Theros and I've settled on a monoblack goodstuff with a Pestilence theme - with Pestilence, Withering Wisps, Crypt Rats and Thrashing Wumpus. I've got Whip of Erebos and Exquisite Blood in there to allow the pestilent critters grant me hundreds of points of life - which makes Last Laugh a pseudo-pestilence effect also. Every single deck has had a Glacial Chasm for years - but it's particularly useful in this one.

Lastly, Chainer, Dementia Master is probably one of the best value black critters you can run. I strongly recommend him.

At one point I had considered Last Laugh but ended up dropping it for some reason or another. I will have to pick up a Chainer and test both and give them a spin. Another card I had high hopes for was Sickening Dreams. The idea was it would give me something to do with all the excess cards I and try and close out a game but I felt it underwhelming whenever I had it in hand.
Unrelated, i think King Macar, the Gold-Cursed would be a pretty fun commander if we can get some more vehicles and such.