We want to donate the opponent a creature that can pay any amount of life with, then take control of them for a turn and drain them to 0.
To donate your opponent the creatures we use, we run 4x Bazaar Trader with 4x Boggart Harbinger as backup.
We can pay as much life as we want to either Wall of Blood or Carrion Howler - we run 4 of both.
Next, we need to take control of our opponent's turn. There's only a few cards that do this, but enough for our purposes. Either Mindslaver or Worst Fears will do the trick. Mindslaver is more reasonable to cast, so 4 of those. We'll run 3 Worst Fears, as well.
Now, 6 mana is a lot in modern, and 8 mana is just a ton. We could use the Urzatron lands to get there, but we probably don't have enough slots left in the deck to get there reliably. What we do have, though, are swamps, for the black cards we need to cast. Crypt Ghast might be able to help us get to the higher amounts of mana we need to have the combo go off.
We'd like some disruption to our opponent's strategy, as well; we're not going off on turn 4, and we have to be ready for the decks that are. So, maindeck we'll run 3 Lightning Bolt, 3 Dismember, and 3 Thoughtseize. We'll put more matchup specific hate in the sideboard.
Now for the mana base. 4x Blood Crypt is pretty much a no-brainer - taps for either of our colors, and has an option to come in untapped. It even works off of our Crypt Ghasts. We'll want 4 Bloodstained Mire to fetch them or our basics. Also, let's throw in 2 Urborg, Tomb of Yawgmoth to help with the Ghasts. Auntie's Hovel works nicely with our 8 goblins, so 4 of those seems fine. And lastly, 4 basic Mountains and 6 basic Swamps - more swamps because of the Ghasts again, of course.
Now, we want a sideboard for dealing with popular strategies. 4 Spellskite is pretty much a must, I think. We need them for the Splinter Twin matchup for sure, at the very least. 4 Ingot Chewer and 2 Shatterstorm will play nicely against affinity, or honestly anything depending on artifacts. 2 Blood Moon will punish greedy mana bases, and keep our own fairly safe. Lastly, 3 Relic of Progenitus for hating on graveyard strategies.