Better Than One

A person outside the game becomes your teammate. (Choose any number of cards in your hand, on top of your library, or on the battlefield under your control. Those cards become your teammate's hand, library, and permanents, respectively.)

Enter the Dungeon

Players play a Magic subgame under the table, starting at 5 life and using their libraries as their decks. The winner searches their library for two cards, puts those cards into their hand, then shuffles.

Shahrazad

Players play a Magic subgame, using their libraries as their decks. Each player who doesn't win the subgame loses half their life, rounded up.

The Countdown Is at One

Players play a Magic subgame, starting at 1 life and using their libraries as their decks. For the rest of the main game, if a source would deal damage to a player who didn't win the subgame, it deals double that damage to that player instead.

Tug of War

Players play a subgame starting at 5 life and with up to three permanent cards with different names from their main-game library on the battlefield. As the subgame ends, the winner chooses one of the cards they put onto the battlefield as the subgame began and puts it onto the battlefield rather than shuffling it into their library.

Black Lotus

{T}, Sacrifice Black Lotus: Add three mana of any one color.

Blacker Lotus

{T}: Tear Blacker Lotus into pieces. Add four mana of any one color. Remove the pieces from the game.

Karn Liberated

+4: Target player exiles a card from their hand.
−3: Exile target permanent.
−14: Restart the game, leaving in exile all non-Aura permanent cards exiled with Karn Liberated. Then put those cards onto the battlefield under your control.