A Burning World
Disclaimer: The characters/worlds of Buffy, Fallout 3 and Fallout New Vegas belong to their respective owners. If you recognize it I do not own it.
Timeline: pre/current Fallout 3/New Vegas universe and post Twilight in Buffy.
Notes: There will be spoilers, anywhere from season 1 of Buffy up to Twilight as well as most of Fallout 3.
She remembered when the bombs fell, when Twilight had finally gained enough influence in the government to stage the last world war. She remembered when Twilight, in an effort to eliminate ‘the slayer threat’, talked the worlds leaders into launching their nuclear missiles and achieved what demons had not been able to in all the time that mankind had tread the surfaces of earth. The world, as it had been, was ended.
What small numbers of demons survived mutated, the rare human that lived through the bombs, and the radiation that followed, goulified or reverted to their earlier primal roots. Humans became as wild as the demonic animal hybrids that roamed the wastes of Earth. The slayer line returned back to one from the many that had been activated by the white witch in the battle of the first. But this was not the last of Twilight; they had planned even for this.
In the time leading up to the final days of the war between the slayer forces and Twilight she had not paid enough attention to media. Outside of its coverage of the increasing hostilities towards slayers it had held little of interest to her. The new nuclear trend had not caught her interest, just another fad that would pass with time. Thus the fact that a new corporation, Vault Tec, had begun to sell tickets to fallout shelters had escaped her notice. When she later discovered that key members of Twilight had all reserved places within these shelters she was not surprised however.
And so the world burned and the slayers went from many back to its origin of one. Buffy, the last of the vampire slayers, wandered, killing the demon hybrids that she stumbled upon and waiting for the day that the descendants of Twilight would leave their vaults and she could take her revenge.