Nirboset, The Dreamer

Flying, defender, indestructible
Whenever Nirboset, The Dreamer leaves the battlefield, all players lose the game.
Whenever a player would draw a card, they
choose a card in their graveyard instead and flip
a coin. On heads they put it in their hand, on tails
they mill a card and choose a card with a different
name. If a player mills all their library this way,
they lose the game.