Damnation by paying its mana cost, destroying all creatures.
Cast
Biorhythm by paying its mana cost, causing each player's life total to become equal to the number of creatures they control.
Each player that controls no creatures loses the game due to having 0 life.
Notes
You will also lose the game if you control no creatures as
Biorhythm resolves, causing the game to end in a draw. You can avoid this by putting a creature onto the battlefield after step 1, or by having an indestructible creature. Similarly, any opponent who controls an indestructible creature will not have 0 life or lose the game.