Xantcha, Sleeper Agent Combo (EDH / Commander)

Original Deck: https://tappedout.net/mtg-decks/xantcha-combo-control/

Paradox Engine and Experimental Frenzy - Paradox Engine works like it does in a Thrasios, Triton Hero deck : every time you can net 3 mana on a spellcast, you can draw another card with Xantcha, and keep casting stuff to go through your whole deck. This is trivially infinite with Sensei's Divining Top and 4 mana worth of mana artifacts, since you pay 1 for the top, tap the top to draw and put the top back, then pay 3 with Xantcha to draw the top again. You can draw your whole deck this way - except you can really only draw enough to kill the player controlling Xantcha, then Xantcha comes back to you. At this point you want to kill Xantcha and recast her to kill another player. Experimental Frenzy is also very strong here. If you have Xantcha out then you can always pay 3 mana to skip past anything on the top of your library that you can't cast, and then just cast the cheap stuff from the top of the library. In either case you eventually get more and more rocks into play making each spellcast more efficient with the Paradox Engine.

Worldgorger Dragon (WGD) - The combo with a Worldgorger Dragon in the graveyard and an Animate Dead generates infinite mana by removing all your permanents from the game, returning them all untapped, giving you time to tap them, and then repeating this process. This also has a cool interaction with Xantcha, since when one opponent dies with Xantcha in play the control-changing effect ends and Xantcha returns to your control. This would normally be sort of inconvenient, but if the WGD loop is on-going, Xantcha will get exiled along with everything else you control and return, and when she does, she'll enter under someone else's control. This is basically the main combo and it can go off pretty quickly. If Xantcha is in play, this whole combo can go off at instant speed with Necromancy and there's no need to even have another creature in a graveyard to end the WGD loop. You just win at instant-speed. You can even set it up entirely at instant-speed with Entomb and Necromancy.

Waste Not - This card is awesome. Super powerful. But mostly just with Magus of the Wheel and stuff like Sire Of Insanity, Burning Inquiry, and Death Cloud. Note that Waste Not is better here with Xantcha in play since if someone discards a land and you get 2 black mana at a bad time, like during someone's end step, you can combine that with 1 more mana to draw a card with Xantcha.

Yawgmoth's Will and Past in Flames - lets you replay stuff out of the yard, particularly all the red and black rituals. And tutors. Which there are a lot of. Past in Flames is a good Entomb target if you can't use the Worldgorger Dragon. If you can fill up the yard you can just go off with rituals and tutors.

Sire Of Insanity - This makes everyone discard everything, which then makes them spend mana on Xantcha, since they have nothing else to do. It is basically asymmetrical since we'd rather play out of the GY with Past in Flames or Animate Dead or off the top with Experimental Frenzy. This is basically Plan B: Make everyone discard everything then play out of the yard and dump mana into Xantcha. If you can Reanimate the Sire Of Insanity on turn 1 or 2, you should definitely do that.

Bloodchief Ascension - This gets turned on pretty trivially by Xantcha's ability, and Xantcha's requirement to attack each turn. It's decent for the extra damage that accumulates with this and Xantcha's ability, which leads to a pretty fast clock. And it also functions as the secondary wincon.

Bonus Round - This card is insane with rituals, tutors, and card draw, which there's a lot of in Red/Black. There's a specific line that starts with 5 mana and Bonus Round and a tutor and a ritual. The tutor can be any 1 of Demonic Tutor or Dark Petition and the ritual can be either Dark Ritual or Cabal Ritual. First:

Play Bonus Round.
Then play the ritual (let's say Dark Ritual), making six mana.
Play the tutor (let's say Demonic Tutor). Use the tutor and the copy of the tutor to get the other ritual and the other tutor (Cabal Ritual and Beseeche the Queen).
Cast Cabal Ritual making 6 more mana.
Cast Beseeche the Queen getting Dark Petition and Lion's Eye Diamond.
Cast Dark Petition (making 6 mana) hold priority and crack the Lion's Eye Diamond. Tutor for Seething Song and Past in Flames.
Cast the Seething Song (making 10 red mana).
Cast Past in Flames (6 red mana left)
Then re-cast Bonus Round from the yard (which gets copied once), meaning that three Bonus Rounds have resolved and you now get to copy each spell 3 times for a total of 4 casts.
Recast Seething Song (making 20 red mana).
Recast Cabal Ritual (making 20 black mana).
Recast Dark Petition (making 12 mana) and tutor for any win condition.
Kill - The cleanest thing is to get Fiery Confluence, cast it doing 6 damage to each opponent, with 3 more copies it does a total of 24 damage. Then recast Past in Flames from the graveyard to give flashback to Fiery Confluence. Recast Fiery Confluence for another 24 damage to all opponents. If that's not enough you can finish people off with Death Cloud or Rolling Earthquake.

This can also work if you have enough mana to just cast the Bonus Round and a tutor with enough left over to cast a ritual: Bonus Round + Tutor + 1 Black mana. Or you can also do it with Seething Song first to make enough mana to cast Bonus Round plus a tutor.