In the world Olympus Coliseum there is the Underworld, within the Underworld is Tartarus.
Tartarus is the ultimate prison and the deepest pit in the Underworld. It is here that Cronus was cast into eternal damnation by his son Zeus. But Cronus waited and soon enough Maleficent came seeking his help. She struck a deal with Cronus, Maleficent could summon up all his monsters and set them on the keybearer and throw all the worlds into everlasting darkness. In return Cronus would be released from his prison and seek revenge against the Olympian gods.
So now all the beasts in Tartarus and all the Heartless are ravaging the worlds. And here the story begins.