In the Guyver: Bioboosted Armor anime/manga series, X-Day is the event when the secret society called Chronos initiates their plan to take over the world. For years, Chronos has been turning human beings into Zoanoids, creatures engineered as biological weapons, and planting Zoanoid agents in key government and military locations across the Earth. On X-Day, the Chronos overlords - the Zoalords - use their telepathic abilities to command all of their Zoanoid sleeper agents across the globe to reveal themselves simultaneously. The result of this is a short but brutal war that ends with the collapse of all world governments and militaries, forcing mankind to surrender to the rule of Chronos.
Following Chronos' rise to power, all previous international borders are dissolved and the world undergoes a tremendous social upheaval. Poverty and disease are quickly eliminated and Chronos introduce Zoanoid optimization as a public service. While Zoanoid processing isn't compulsory, Chronos strongly encourage it through the media and many people agree to undergo the process since Zoanoids are immune to all known diseases. What the public are not made aware of, of course, is that Zoanoids are all genetically programmed to obey the commands of the Zoalords, meaning that Zoanoid optimization also eliminates free will.
Although Chronos controls the world, they are still threatened by the Guyvers Sho Fukamachi and Agito Makishima.