The End of the World was a vision of the future foreseen by Dinah Alcott and prepared for by multiple parties.
While imprisoned by Coil while the Slaughterhouse Nine were attacking Brockton Bay, Dinah informed him that, should Jack Slash escape the city, the world would almost certainly be destroyed within a few years. He informed the heroes of this.
The world was set to end somewhere near a specific date in two years time if Jack survived. If he did not, it would still probably occur, but not for many years.
The death toll varied, but the vast majority of the population would be wiped out, leaving scattered survivors.
The heroes Dragon and Defiant set out to destroy the Slaughterhouse Nine. When they resurfaced as the The Slaughterhouse Nine-Thousand, Weaver led the attack on them, which deliberately excluded heroes like The Triumvirate who might be powerful enough that they could trigger the end of the world single-handedly. Saint, theorizing that her increasing freedom to follow the Nine was what led to the end of the world, neutralized Dragon with the Deployment of Ascalon.
Jack Slash threatened to order his army to kill as many people as possible, which he suggested might be the prophesied end of the world, if the heroes broke the rules of his game with Theo Anders. Weaver killed Aster because she worried the child might be about to trigger with a power that would cause the end of the world.
For full article, see Gold Morning.