Disclaimer: i obviously don't own anything Kingdom Hearts, except for the idea for this fanfic. I don't know how much of this is actually accurate to the KH story line, but i hope at least some of it is close. enjoy
SLAM! Riku threw his weight against the Door to Darkness, along with King Mickey, sealing the Heartless inside and saving all of the various worlds in the universe from being overrun. Riku grinned ruefully at the King. "We did it," he said. "Sora and Kairi are safe."
Mickey twirled his Keyblade. "But we're still in danger Riku," he said. "Stand and fight!" The two of them spun and brandished their swords at the advancing Heartless, who just laughed evilly and threw themselves forward. Riku and Mickey yelled their battle cries and sprang forward, their blades spinning with almost inhuman speed. Heartless vanished left and right in clouds of smoke that made it hard for Riku and Mickey to see, but they just stood back to back and struck at anything that moved within their reach. They never seemed to tire, as if when they sealed the Door to Darkness they sealed away their mortal attributes, becoming much stronger than they ever would have.
It seemed as if they fought for hours on end, even days, but it was impossible to tell in the eternal darkness where they only light came from the King's Keyblade and the creepy blowing eyes of the Heartless.
Finally the waves of Heartless began to decrease in size, though the two fighters were sure that it wasn't the actual numbers decreasing, the Heartless were becoming slightly more cautions. "Why are they holding back?" Riku panted. "They have virtually unlimited numbers!" He lobbed off the head of a Heartless who strayed too close and then disappeared. "Come on you cowards! I'm not afraid of you! Let me hone my skills against your ranks! I've got all the time in the world!" He charged forward and cut his way to a huge, armored Heartless, who he easily defeated by slicing its legs and stabbing its back when it fell. The loss of on of their champions seemed to scare the Heartless, and they fell back even further, out of the circle of light cast by the King's Keyblade. There they lingered, a massive, roiling black mass covered with glowing eyes. Every once in a while a few would venture forward, only to be quickly dispatched. Riku and Mickey sat down and drew several deep breaths to steady their heartbeats.
"I think you're mistaken about the number of Heartless there are, Riku," the King said.
Riku looked up. "What do you mean your majesty?"
"Well, there do seem to be a lot of them, so many that they seem to unlimited, but I believe their numbers are limited to the number of people in the universe with evil in their hearts. Obviously this would include pretty much everyone because no one is perfect, but not everyone has the same amount of evil in their hearts. Some people have hearts as black as a lump of coal, and it is they who create the stronger Heartless, such as the armored one you just defeated." Riku nodded. Easy to understand so far. "Then there are others who have some bad intentions but are not evil in nature. They create Heartless of medium strength. Then there are everyday people who just want to live their lives but cant help making a few bad choices. They create the smallest Heartless, the ones who are the easiest to kill." He lashed out at a random Heartless who was getting too close.
"I guess that makes sense," Riku said. He was silent and thoughtful for a moment, then said quietly, "I guess the Heartless I make must be pretty bad."
Mickey nodded solemnly. "Yes. Do you remember Ansem? Well that veing isn't actually the real Ansem. It is the evil part of the real Ansem that got loose and saw in you a quality it could exploit, that is, your growing hatred of being cooped up on the Destiny Islands all your life. The evil spirit that originated from Ansem caught onto this and drew upon it and became powerful enough to assume a form. Not a proper form of flesh and bone, but form nonetheless. His body composition is like that of a Heartless."
Riku held up a hand. "Okay, so hang on a sec. So are you saying we are destroying people's bad intentions and that if we could find the fake Ansem and destroy him then both mine and Ansem's hearts would be free of evil?
Mickey shook his head. "No, I don't know exactly what happens but I don't think they just go away. My guess would be that they eventually regenerate at the core of this black world and make their way back here to try and escape through the Door to Darkness. This I doubt that destroying the Heartless Ansem would do much good other than delay his plans. You would need to do something more, and it would require a team effort, with you being a key part of that team. I would think that it would involve Ansem too, because he is affected, and Sora because he is the Keyblade bearer and it's his job to do such things."
"Oh great," Riku chuckled. "Now I'm relying on Sora instead of the other way around like it was back on the islands. What is this world--" he stopped himself, "or rather, this whole universe, coming to?"
Mickey looked around. The Heartless were stirring. "Let's look for Ansem so we can figure it out."
"After we kill a few more Heartless that is," Riku said, drawing his sword.
"Of course," Mickey said. They got into their battle-ready stances. "These ones are for you, Sora and Kairi," Riku whispered, then cried in unison with the King as he threw himself into the fray with renewed vigor, knowing now that he had a chance against the Heartless and the darkness inside him.