the heartless
The beings that you must fight in all of the worlds are called the Heartless. They are beings that are pure darkness, and they thrive off of the darkness in the heart of the people. They are a widespread disaster, and you must destroy them. They originate from Hollow Bastion, but it is possible that they have lived in the hearts of all worlds, hibernating until Ansem started his experiments. It seemed that after Ansem's disappearance, they multiplied uncontrollably. The experiments of a madman gone horribly wrong. They swallowed the heart of the planet, and the people were spread throughout the worlds. While the people were spread, so were the heartless. Soon after, many worlds starting experiencing similar phenomena.