Planescape: Fallout/The story so far/Backstory

Everyone agrees that for many years in the distant past, the Blood War raged across the planes. The Tanarri(chaotic evil demons) and the Baatezu(lawful evil devils) fought each other from their respective home planes of the Abyss and Baator for the shape of what evil was to become. It ranged far across the planes, but others generally declined to intervene, in the fear that the winning side would then turn their attention on their own home planes.

What brought it to a head is less clear: some say that the Godsmen Foundry in Sigil developed a weapon for the Baatezu, and, fearing they would win with it, gave another to the Tanarri, hoping neither side would use it for fear of mutually assured destruction. Others say that necromancers developed a spell that would kill the threads that stiched together the very fabric of reality. Others say that it was Coaxmetal himself, the giant golem forged of weapons, that forged and wielded a plane-destroying blade indiscriminately against the universe, and that the Blood War denizens urged him on.

Whatever the cause, the planes began to crumble and Sigil disappeared—no portal that used to lead there worked any more, and it was no longer seen suspended above the Great Spire. Forseeing the end, some groups of planar mages saw one way through the destruction: to perform the same ritual that Sigil apparently had done, and disconnect entire cities and encase them in their own pocket dimensions, there to wait out the fall of the planes.

'Web' was one such experiment: a series of islands, floating in space and connected with a series of ropes and bridges, each island hailing from a different home plane.