"A heart that has has been overcome by darkness. The creatures seek only to multiply and will actively seek out those with a high amount of darkness within their hearts. The more powerful the darkness within one's heart, the more powerful the heartless that comes from it. For the most part, they are mindless and act on instinct, but those with enough darkness within themselves are capable of controlling them. ...To a point. If they slip up, the consequences are dire." He shook his head slowly, hands folding around the cup of tea--still warm to the touch but quickly cooling. The steam had long since stopped its rise.

"But you need not worry," Marluxia continued to explain. "The ones closest to the city are easily dispatched. You have more to worry from those housed in City Hall than the Heartless, though they seem to go hand in hand. That group is one of the few people who have some measure of control over the Heartless."