First thing, Korlash, Heir to Blackblade is my favorite card. He's not very good, dies easily, not really special at all, but he looks cool and to me, that's what matters.
He also has this cool ability called grandeur (wish they would have done more with it), once you get this bad larry to the battlefield, you can activate his grandeur ability. Once activated, you discard other Korlash who are in your hand to get lots of swamps onto the battlefield free of charge, I use it to tutor my shock lands Overgrown Tomb and Godless Shrine and later just regular swamps.
Now, you may be asking, why do I need these shock lands for a mono black midrange deck? Well that brings me to one of the other cards. Mastermind's Acquisition. MA is a tutor card, basically a diabolic tutor but it gives you the option to tutor for a card in your sideboard. So for us to have an "effective" sideboard, we must splash in some colors. MA allows you to deal with threats as they are presented to you, without having to wait to sideboard. It also allows us to grab Torment of Hailfire (which will be your go to win con). Hailfire basically ruins everyone's day except yours. The guy in front of you, to the left, to the right, around the block, don't matter.
Of course, before you tutor for ToH, you're gonna want lands, and lots of them, so that's where ol' lashy-poo comes in. Early game, we want to tutor for our Heir's to Blackblade (with profane tutor, transmuting House Guard, or even MA) to drop at least two kiddies off at the pool (with grandeur) to get us four swamps in play. If done correctly, you can have anywhere from 6 to 10 swamps at the end of turn 4, which will help with ToH's destructive power. Cabal Coffers helps us take our opponent straight to value town to dial up the ol pain meter on the ToH.
Now, sometimes, depending on what your up against, ToH or maybe even ol' Lashy might not be suitable for what we need to accomplish. We can use Dimir House Guard to tutor for 4 cmc cards (which is like, the whole deck) and use profane tutor to grab anything else you need. Use MA to grab something from your sideboard to deal with a threat as you see it.
Also x2 Phyrexian Obliterators because fk you, fk your cover, and fk the guy behind you!
Now, once we start getting late game, and you still haven't killed off your opponent, that's where our devotional to el swamp comes in. Introducing (you already know) Gray Merchant of Asphodel. This can save your a$$ with either the killing blow or just some of that wholesome life juice so you keep kicking for a turn or two. For him to be the most effective you're gonna wanna play your Phyrexian Arenas and Planeswalkers, your Bloodghast (which conviently comes back to life every time you wipe the board then drop a land), your Messengers (who can also come back to life), and (ofc) your Obliterators for maximum life loss (and gain).
I've playtested this deck only in MTGO but it has been decently successful, my only real big problems are dealing with heavy control decks, but that's what the sideboard is for!
Obviously, there is much more that you can do with this deck, but I can't share all the secrets
