Sunder

The Sunder was an apocalyptic event over a thousand years ago where the necromancer Indrik sought to rule over the world by raising an army of the undead. He used a drill and dark magic to reach the planets core, nanauruat, where the dead reside. The necromancer had a brother Sirin who made a coventant with the old gods giving him the power to stop his evil brother. Sirin fought the undead army and destroyed the drill before it was too late, but the drill tip had already made contact with the core, tapping into its power fracturing and destroying the world. The fractured bits created giant islands where the survivors created a new life for themselves. These floating islands still orbit the core, remain perfectly in tact, it's uncertain how many people survived, or how many other islands can support life, they are just to far away to reach.