The combo this deck is built around is infinitely sacrificing and recurring creatures, and using those ETB/LTB triggers to win. You don't need to combo off to win, but it is a very efficient way to do it. There are a lot of different ways to put together this combo, so be prepared to get creative!
There are four components to this combo. Cards are listed loosely from best to worst for how well they fulfill their respective part of the combo. The best ones fulfill more than one part.
Part 1: Self Recurring Creatures
All of these creatures have the same mana cost to recur them, making them generally interchangeable but some do have restrictions on when they can be used that are important to consider.
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Reassembling Skeleton This is generally the best option. It is consistent and unrestricted, so you have the most choice in when you combo off.
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Bloodsoaked Champion Has the benefit of being 1CMC rather than 2, but the combat restriction means you can only combo off on your turn, which can be relevant.
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Tenacious Dead Like Bloodsoaked Champion it is 1CMC, but it is the least consistent because you have to have the mana available to recur it right when it dies. This means you can't start the combo with it in your graveyard, and you have to find another effect to bring it back if it ends up there.
With some tweaking these are some other options you could run: Nether Traitor, Gravecrawler, Oathsworn Vampire
Part 2: Sacrifice Outlets
Any of the outlets in the "Free Sacrifice Outlet" category will work for this part of the combo.
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Ashnod's Altar Overall the best sacrifice outlet in the deck, and the best for this combo. Because it produces mana from sacrificing, all you need is something to filter some of it to black (discussed in the mana section).
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Altar of Dementia This is another strong option because it can act as both a sacrifice outlet, and the way to win off the combo (milling your opponents out).
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Carrion Feeder 1CMC and in very limited circumstances can act as a win condition for the combo.
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Viscera Seer 1CMC and useful for fixing your draws. Outside of the combo it is usually better than Carrion Feeder
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Fanatical Devotion Allows you to protect your other combo pieces from removal. If you don't care about getting the combo off early this can be a better option than Carrion Feeder and Viscera Seer.
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Hidden Stockpile Costs 1 mana to sacrifice a creature, meaning you have to produce more mana to go infinite with this than any of the free sacrifice outlets, but it can work in a pinch.
Part 3: Mana
All the self recurring creatures this deck runs require one generic mana and one black to return them from the graveyard to the battlefield. There are a few different ways to put this part of the combo together, but as long as you are able to produce two mana (at least one being black) from every loop you are good to go.
One of these two is required because they are the only way to produce an arbitrary amount of black mana.
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Pitiless Plunderer Treasure tokens can be sacrificed for mana of any color, so this is a great option.
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Bog Initiate This can filter any amount of colorless mana that you have to black.
These can be used to support one of the above two.
Teysa Karlov can also be an important tool for putting this part of the combo.
Requiem Angel or any other token producer can make an additional token to sacrifice to get a second copy of Pitiless Plunderer's effect.
Here are a couple different ways to put together this part of the combo:
If you put this part together in a way that produces more than two mana per loop and you don't have something to use this combo to win, you can just use the infinite mana to cast whatever else you want to. This deck doesn't run any, but if you run cards like Exsanguinate you can also use this infinite mana directly to win.
Another fun optional piece of tech for this combo is Embalmer's Tools. It reduces the recurring cost to just {B} for Reassembling Skeleton and Bloodsoaked Champion which is equivalent to creating an additional mana per loop. It doesn't really have any application in this deck outside of recurring those two specific creatures which is why I haven't included it, but if you want to go all in on the combo its an option.
If you have one, Phyrexian Altar can also be used for this part of the combo.
Another option: Pawn of Ulamog can fill the same role as Sifter of Skulls.
Part 4: Winning
Like the mana section there are a variety of options for how to turn this combo into a win, but generally this part is a bit more straightforward.
The first option is using direct damage. Any of the below will be effectively equivalent when doing this combo infinitely so I've just listed them by mana cost, but they do have some individual nuances if you aren't able to go infinite yet.
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Zulaport Cutthroat Cheap and hits the whole table, it is a 1/1 so its not hard for it to get removed.
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Blood Artist It targets which can be a good political tool. Same downside at Zulaport Cutthroat, easily removable.
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Cruel Celebrant Slightly more restrictive casting cost, but also more durable.
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Corpse Knight Doesn't get doubled by Teysa
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Falkenrath Noble Basically a tougher Blood Artist
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Syr Konrad, the Grim Triggers when creatures are recurred as well as when they die, and also triggers off milling. Highest potential damage output in a non-infinite context.
I don't run it anymore, but Vindictive Vampire also goes in this category.
The next option is milling. The main card that does this is Altar of Dementia. It is a good backup to the direct damage route, but you do have to wait for your opponents to draw from an empty library to die, which depending how competitive a table you are at can be a big risk. Altar of Dementia
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Syr Konrad, the Grim
is a way you can pull off an immediate win when there is a Hushbringer type effect suppressing on-death effects. Syr Konrad, the Grim can also mill out opponents if you can produce infinite black mana, but his mill is symmetrical so you risk drawing out as well. That said, if you are playing against other creature heavy decks and a Hushbringer is preventing Konrad from pinging on death, you can use Konrad's mill to ping everyone, but this requires the total amount of creatures left in libraries to be greater than or equal to the highest life total among your opponents, which isn't always the case.
Plan C is creating infinite tokens. Any of the cards that create tokens when another creature dies can do this, but this deck doesn't have a way to give the tokens haste, so you either have to make the infinite tokens on the end-step before your turn (not possible with Bloodsoaked Champion), or wait a full turn cycle to untap with them which puts a huge target on you. Teysa Karlov does give them lifelink and vigilance, so they can attack and block quite profitably if you aren't able to win with them. That said, these cards are your options for this win condition:
As mentioned above, although it isn't a great option you can use this combo to make an infinitely large Carrion Feeder and attack with that to win. It doesn't have trample or evasion, you can only kill one opponent at a time with it, and it doesn't have any protection from removal. That said winning with a huge zombie would be pretty sweet.
If I missed any ways in this deck to win off this combo, let me know.
Important Note: Your card draw outlets can kill you.
The card draw on Dark Prophecy, Grim Haruspex, and Midnight Reaper are not 'may' abilities, so if you go infinite with those on board you can kill yourself by drawing your entire library if you have less cards than your opponents have health. Both Dark Prophecy and Midnight Reaper deal damage to you when your creatures die, so you need to make sure you have a way to cancel out that damage or not have those cards on board. Midnight Reaper can be sacrificed when you no longer need the draw, but Dark Prophecy needs to be cancelled out. Blood Artist, Cruel Celebrant, Zulaport Cutthroat, and Falkenrath Noble all gain you life, but Corpse Knight and Syr Konrad, the Grim don't.