The Apocalypse

The Apocalypse is a quest in the Fallout: New Vegas add-on Lonesome Road.

Quick walkthrough
This quest appears after you deal with Ulysses and decide the fate of the nuclear missile. Choosing to nuke either territory, instead of cancelling the launch, will begin this quest and fail The End. Leaving the temple, and watching the DLC ending ends the quest. A real time nuke explosion is seen on the horizon, in the general vicinity of whichever territory you targeted (or both, if you chose to nuke both Legion and NCR).