NOTE: I only own a single copy of Sheoldred, the Apocalypse that I was fortunate enough to pull from a set booster (remember those???), but if I could, I would definitely play 2 of them-- but I can make a solid argument that my single copy is plenty with this current build.
Combo Creatures
This combo has been explained in the Combo Strategy section, but here's the brief summary: get Unstoppable Slasher and Bloodletter of Aclazotz onto the battlefield, deal damage with the Slasher so the opponent loses half their life and the Bloodletter doubles that life loss to end the game on the spot.
Unstoppable Slasher is a very, very interesting Magic card. at , we get a 2/3 with deathtouch which would ordinarily be pretty meh. However, the additional novel-length text that WotC now adds to every card states that our opponent loses half their life (rounded up) if this creature does combat damage. A single hit could do a lot more than the 2 damage that the power of this creature indicates!
Bloodletter of Aclazotz is a good card by itself-- a 2/4 flier that doubles any opponent life loss during our turn is just a quality card. It effectively doubles the power of all of our creatures. Yes, the can be rough, so we have to go virtually all-in on our mana, but I think it's worth it. Oh, and the combo thing. Yeah, that's pretty good, too.
The Planeswalker
Liliana of the Veil is just such a strong card. It basically gives us an additional removal spell, and if left unchecked will crush our opponent's board.
The "Fair" Creatures
Deep-Cavern Bat gives us some early hand control and a flier, so not bad. This can easily disrupt our opponent's game plan, or at least slow it down by a turn.
Gix, Yawgmoth Praetor draws us cards whenever we do combat damage, and it can serve as a place to use mana late game if we're grinding out a long game. Really, though, the card draw is pretty incredible.
Glissa Sunslayer is a 3/3 for with a lot of upside and flexibility. Did I mention the firststrike AND deathtouch? That's a ridiculous combination on a single card!
Mosswood Dreadknight was a pretty good card. We can use it to dig through the deck before we get some of our other card draw engines out (e.g., Gix, Yawgmoth Praetor, Glissa Sunslayer), then cast it and use it to saddle the Caustic Bronco, or just swing in with the trample. It's a really good card in a "pile of good cards."
Sheoldred, the Apocalypse is arguably the best card in Standard. It's so powerful. The 5 toughness makes it hard to kill, the deathtouch gives our opponents pause, and the life gain/drain can be backbreaking for opponents.
Venerated Rotpriest is really here as an additional 1-drop. Most Golgari builds only have Duress as the 1-drop. I prefer to have just a bit more available on turn 1, so I added this overlooked gem. We have a lot of creatures, and with Venerated Rotpriest we make our opponents pay (in poison!) to target them with removal.