This deck uses Heartless Summoning to force death triggers to occur.
The most notable win condition of this deck is the infinite combo with Falkenrath Noble and 2 Myr Retrievers. That's a 4-card combo, though; so we can't always rely on this; it can't be our only avenue.
With Heartless Summoning out, our creatures have efficiency in spades. Perilous Myr hits something for 2 for free, and once you have one in your grave, so do Myr Retrievers. Black Cat goes from being a mildly annoying 2-drop to a 1-drop random discard sorcery.Golgari Thug at the worst will give you some (slow) recursion if you absolutely need it, but he's great at filling up your graveyard with tasty two drops.
The only 1-toughness creature here that sticks out as not particularly useful with Heartless Summoning is Blood Artist. That brings me to another main feature of this deck: Immortal Servitude. All those derpy little creatures we just filled our graveyard with? for five mana, they're all back for round 2! And this time, Blood Artist is going to leave a mark, if not win the game outright.
Dark Prophecy/Harvester of Souls on top of this adds frighteningly powerful card draw to the deck.
2 Myr Retrievers gives you the ability to draw as much of your deck as needed (ie draw until you hit noble, play for 2, win game).
Golgari Thug goes from mediocrity to a total workhorse. By stacking the triggers appropriately, you can grab any creature from your graveyard, play it, dredge the thug back to your hand, and repeat as needed.
Golgari Thug + Perilous Myr
B, Pay 2 life: Deal 2 damage to target creature or player. Put the top 4 cards of your library into your graveyard.
Golgari Thug + Black Cat
BB, Pay 2 life: Target Opponent discards a card at random. Put the top 4 cards of your library into your graveyard.
Golgari Thug + Myr Retriever + Perilous Myr
B, Pay 3 life: Deal 2 damage to target creature or player. Put the top 4 cards of your library into your graveyard. Draw a Card
The best part about the Thug is that your options expand with your graveyard, making it all to easy for the thug to troll along until you get infinite card draw with Myr Retriever, or until you can grab a Falkenrath Noble from the graveyard and win the game on the spot.
There are a few one-ofs in the deck. First, there's Bloodghast. Bloodghast is nice because with Heartless Summoning out, every land drop will trigger whatever death effects you might have on the field. He's also nice because he requires such little investment; with Golgari Thug dredging like it's going out of style, he'll probably show up into the graveyard sooner or later.
Second, we have Grave Pact. I like Grave Pact, but I don't want to be tapping out for it unless I have some free Myr in my hand to make immediate use out of it.
Harvester of Souls is part of a 3-1 split with Dark Prophecy. Dark Prophecy is usually preferred, as it's cheaper, but Harvester of Souls can be grabbed from the graveyard by Golgari Thug.
On the tutor side of things, Beseech the Queen is quite useful. It's best used after a Heartless Summoning to tutor up the free myr to kick the deck into high gear, and can grab other combo pieces in a pinch. Aside from Nobles, Harvester, and Grave Pact, everything is tutor-able on turn 3, and you should be able to start the ball rolling after a turn 3 beseech.
