Cast Chthonian Nightmare by paying {B}.
When Chthonian Nightmare enters the battlefield, it triggers, causing you to get {E}{E}{E}.
Activate Chthonian Nightmare by paying {E}, sacrificing Greedy Freebooter and returning Chthonian Nightmare from the battlefield to your hand.
When Greedy Freebooter dies, it triggers, causing you to scry 1 and create a Treasure token.
Resolve the Chthonian Nightmare ability, returning Shambling Ghast from your graveyard to the battlefield.
Activate the Treasure by tapping and sacrificing it, adding {B}.
Cast Chthonian Nightmare by paying {B}.
When Chthonian Nightmare enters the battlefield, it triggers, causing you to get {E}{E}{E}.
Activate Chthonian Nightmare by paying {E}, sacrificing Shambling Ghast and returning Chthonian Nightmare from the battlefield to your hand.
When Shambling Ghast dies, it triggers, creating a Treasure token.
Resolve the Chthonian Nightmare ability, returning Greedy Freebooter from your graveyard to the battlefield.
Activate the Treasure by tapping and sacrificing it, adding {B}.
Repeat.

Infinite LTB

Infinite ETB

Infinite sacrifice triggers

Infinite death triggers

Infinite scry 1

Infinite storm count

Infinite energy counters

Chthonian Nightmare

Greedy Freebooter

Shambling Ghast

Thunderscape Familiar

Activate Chthonian Nightmare by paying at most {E}{E}{E}, sacrificing Patron of the Arts and returning Chthonian Nightmare from the battlefield to your hand.
When Patron of the Arts dies, it triggers, creating a Treasure token.
Resolve the Chthonian Nightmare ability, returning another creature card from your graveyard to the battlefield.
Activate the Treasure by tapping and sacrificing it, adding {B}.
Cast Chthonian Nightmare by paying {B}.
When Chthonian Nightmare enters the battlefield, it triggers, causing you to get {E}{E}{E}.
Activate Chthonian Nightmare by paying {E}{E}{E}, sacrificing another creature and returning Chthonian Nightmare from the battlefield to your hand, returning Patron of the Arts from your graveyard to the battlefield.
When Patron of the Arts enters the battlefield, it triggers, creating a Treasure token.
Activate the Treasure by tapping and sacrificing it, adding {B}.
Cast Chthonian Nightmare by paying {B}.
When Chthonian Nightmare enters the battlefield, it triggers, causing you to get {E}{E}{E}.
Repeat.

Patron of the Arts

Activate Carrion Feeder by sacrificing Priest of Gix.
When Priest of Gix dies, Rakdos triggers.
Holding priority, activate Haunted Crossroads by paying {B}, putting Priest of Gix from your graveyard onto the top of your library.
Resolve the Rakdos trigger, causing you to exile the top three cards of your library with the ability to play them until your next end step.
Resolve the Carrion Feeder ability from step 1, putting a +1/+1 counter on it.
Cast Priest of Gix from exile by paying {2}.
When Priest of Gix enters the battlefield, it triggers, adding {B}{B}{B}.
Repeat.

Infinite +1/+1 counters on Carrion Feeder

Exile your library with the ability to play the exiled cards until your next turn

Rakdos, the Muscle

Priest of Gix

Haunted Crossroads

Carrion Feeder

Activate Phyrexian Altar by sacrificing Oathsworn Vampire, adding {B}.
Cast Oathsworn Vampire from your graveyard by paying {B}.
Repeat.

Oathsworn Vampire

Phyrexian Altar

Cast Mourning by paying 2 life if able, or {B} otherwise.
Each creature you control's extort ability triggers.
Resolve the first extort trigger, causing you to pay 2 life if able, or {W/B} otherwise, causing each opponent to lose 1 life and you to gain that much life.
Repeat step 3 for each remaining extort trigger.
Resolve Mourning, attaching it to a creature you control.
Activate Mourning by paying 2 life, returning it from the battlefield to your hand.
Repeat until an opponent loses the game due to having 0 or less life.

Near-infinite lifegain

Near-infinite lifeloss

Near-infinite lifegain triggers

Near-infinite storm count

K'rrik, Son of Yawgmoth

Pontiff of Blight

Mourning

Cast Nameless Inversion from your graveyard, as it is a Knight card, by paying {0} due to Thunderscape Familiar and Morophon, giving a creature +3/-3 until end of turn.
If the creature has toughness 0 or less, the creature dies as a state-based action.
Repeat as desired.

Reduce the toughness of opponents' creatures to 0 when it enters the battlefield

Reduce any number of target creatures' toughness to 0

Haakon, Stromgald Scourge

Morophon, the Boundless

Nameless Inversion

Combos with "Thunderscape Familiar"