crack of doom
1. The Christian Day of Judgment, when God assigns an eternal fate to all individual humans; a sound or signal heralding that day. You may have been acquitted of your crimes by a court of law, but you will have to face the punishment for them at the crack of doom. The sky blackened and lightning flashed violently across the sky, with thunder booming like a crack of doom.
2. By extension, the apocalypse or end of the world, or a signal thereof. Many feared that the Cold War could at any time escalate to nuclear warfare, bringing about the crack of doom.