Activate Blood Celebrant by paying {B} and 1 life, adding {B}.
Repeat step 1 until you have 0 life.
Cast Repay in Kind by paying {5}{B}{B}, causing each opponent's life total to become 0.
Each opponent loses the game as a state-based action due to having 0 life.

Each opponent loses the game

Near-infinite lifeloss for all players

Permanent with "You don't lose the game due to having 0 or less life"

Repay in Kind

Blood Celebrant

Cast Lich by paying {B}{B}{B}{B}.
Lich entering the battlefield, causing you to lose life equal to your life total.
Cast Repay in Kind by paying {5}{B}{B}, causing each player's life total to become equal to the lowest life total among all players.

Infinite lifeloss

Lich

Activate Blood Celebrant by paying three life, adding {B}.
Repeat step 1 until you have three or less life.
Cast Repay in Kind by paying {5}{B}{B}, causing each player's life total to become equal to the lowest life total among all players.
Cast Blood Tithe by paying {3}{B}, causing each opponent to lose three life and gaining you life equal to life lost this way.
Each opponent loses the game due to having zero or less life.

Near-infinite lifeloss

K'rrik, Son of Yawgmoth

Blood Tithe

Cast Doomsday by paying {B}{B}{B}, searching your library and graveyard for Bolas's Citadel, Emergence Zone, Repay in Kind, Lich, and any other card, exiling all other cards from your graveyard and library, and then placing the remaining cards on top of your library in the order previously listed and losing half your life, rounded up.
Activate Commander's Sphere by sacrificing it, drawing a card.
Cast Bolas's Citadel by paying {3}{B}{B}{B}.
Play Emergence Zone from the top of your library.
Activate Emergence Zone by paying {1}, tapping, and sacrificing it.
Cast Repay in Kind from the top of your library by paying seven life.
Holding priority, cast Lich from the top of your library by paying 4 life.
Lich entering the battlefield, causing you to lose life equal to your life total.
Resolve the Repay in Kind, causing each player's life total to become equal to the lowest life total among all players.

Doomsday

Commander's Sphere

Bolas's Citadel

Emergence Zone

Activate Kuro by paying 1 life, targeting any creature not used for this combo.
Holding priority, repeat the previous step until you have 1 life remaining.
Resolve all Kuro abilities.
Cast Repay in Kind by paying {5}{B}{B}, causing all life totals to become 1.
Cast Temporal Extortion by paying {B}{B}{B}{B}.
If an opponent pays 1 life to counter it, that opponent's life total becomes 0, they lose the game, and you skip the next step.
Otherwise, resolve Temporal Extortion, gaining an extra turn.
Activate Canal Dredger by tapping it, putting Temporal Extortion from your graveyard on the bottom of your library.
Declare Varragoth as an attacker.
Activate Varragoth's Boast ability by paying {1}{B}, searching your library for Temporal Extortion and putting on top of your library.
Go to your next turn.
Draw Temporal Extortion in your draw step.
Repeat from step 7.

On each of your turns, you take an extra turn unless an opponent chooses to lose the game

Varragoth, Bloodsky Sire

Kuro, Pitlord

Temporal Extortion

Canal Dredger

Activate Unspeakable Symbol as many times as possible by paying all but 1 life, putting +1/+1 counters on creatures you control Cast Repay in Kind by paying {5}{B}{B}, causing all life totals to become 1.
Cast Temporal Extortion by paying {B}{B}{B}{B}.
If an opponent pays 1 life to counter it, that opponent's life total becomes 0, they lose the game, and you skip the next step.
Otherwise, resolve Temporal Extortion, gaining an extra turn.
Activate Canal Dredger by tapping it, putting Temporal Extortion from your graveyard on the bottom of your library.
Declare Varragoth as an attacker.
Activate Varragoth's Boast ability by paying {1}{B}, searching your library for Temporal Extortion and putting on top of your library.
Go to your next turn.
Draw Temporal Extortion in your draw step.
Repeat from step 7.

Combos with "Repay in Kind"