Aincrad is a floating fortress high above the ruins of Sendrellar. Created by an alliance of Venir, Lilyth, and Absulcon, it houses all the monstrosities that were once kept beneath Sendrellar in dungeons. Inside, Aincrad is a fully realized sub-plane as large as a continent. This sub-plane has one hundred levels, each with more difficult challenges than the last.
The purpose of this creation was to give mortals the chance to prove their might, grow strong, and take control of their own destinies. It is a place where gods cannot reach... though divine powers still function somehow. Thus, everything that happens in Aincrad is of mortal doing. Anyone who wishes entrance may enter it, though you can only leave by winning.
For you see, Aincrad is a competition. Anyone to reach the 100th level and defeat whatever unknown challenges lie within would, rumors say, be allowed to claim one of the ancient war god Hizos's swords as prize, along with all the power that comes with owning such a relic. A man, or woman, with such power could carve for himself an unforgettable place in history, for good or ill.
Rumors have it that whole cities, even nations, have formed within its extra-planar walls, and that one can find a life inside, even if they don't desire to scale the levels attempting to reach the top. But various leading factions also war with each other in efforts to slow down their opponents' progress.
Thus Aincrad is seen by the world with a mixed light. Some see it as an abomination and blasphemy given physical form. Others see it as an opportunity for mortals to rise above their deific puppetteers. Still others view the fortress as a means to obtain power. Whatever the view, many, many adventurers are lured into its depth. So far, none have emerged.