You must capture the enemy king by your 45th move. If you make your 45th move and the game is still going, you lose.
Doomsday Clockis one of Nerf Chess's secret handicaps. In Nerf mode you choose one of two nerfs before the first move, in secret, and carry it for the whole game.
The board quietly enforces it: any move it forbids is simply never offered, so you cannot break it by accident. Your opponent never sees which nerf you took until the game ends, so every move you make is also a clue they are trying to read.
“The bell was cast the day you were born.”
Affects your kings.