And yes, I know that neither the characters nor the song the story came from (if applicable) belong to me. They are Square's. Yay Square! (and Disney in this case) But the lament/story continued is mine.
Riku no longer knew the difference between night and day. All time perfectly blended together behind the door, with sleep being the only reminder of its passage. Through half-closed eyes, he could see Mickey fighting off the hordes of Heartless. The exiled king was his protector in the uncertain realm of slumber, shielding him from any harm. He owed the mouse nothing though, for he performed a similar task in the opposite situation. It was rather aggravating to watch Mickey sleep, for as soon as he would find an opportunity to take the Keyblade, Riku would be swarmed with the shadows that endlessly plagued the sealed cavern. After what seemed to be ages for both, the Heartless began to slowly decline in number per wave. Life behind the door eventually became little more than snippets of conversation between the allies, followed by short meals of Heartless flesh. More talking would come soon after, ending with a swift and restless sleep. Every day and decreasing in frequency as time passed, an army of monsters would attack, providing enough nourishment to sustain the pair. Mickey wasn't much of an addition to the lifestyle besides a link to sanity, though he did make a decent sparring partner. Their battles often ended in a stalemate, despite Mickey's possession of the Keyblade. Riku's raw power, aided by the lingering traces of the abilities that the darkness had given him, soon became enough to overpower the mouse. He did not want to use his full potential, for that honor belonged to Sora. He decided to stay relatively docile until the king was at his most vulnerable. Riku no longer needed sleep. He would not rest until the key was finally in his hands again...
"C'mon Mickey. You need your rest. Don't worry. You should know by now that all the darkness is behind me. If any Heartless threaten you, I'll get 'em. Now go to sleep already."
Mickey couldn't help but believe Riku's empty words, for the two had seemingly bonded, and soon after fell asleep, Keyblade in hand. The boy's turquoise eyes saw that though the king was youthful in spirit, he was in truth quite old. Among the black of his skin, many scars of battle were vaguely apparent. Riku's respect for Mickey grew ever slightly right then, but all was soon overshadowed by his lust for power. Riku knew that if he wanted the Keyblade, he would have to immobilize or even slay the mouse. His wooden sword would simply not be enough to do the deed. While pondering how to apply his dark powers to the situation, he impaled a passing Shadow with his useless toy. Like most Heartless it barely bled, which made it prime material for consumption, even if a tad disgusting to the tongue. He devoured it with great haste, tossing the tiny bones aside. Wiping the ebony sinews from his mouth, he faced his soon-to-be victim. He rushed to the sleeping body, grabbed the right arm, and twisted it until the bones snapped. Though Mickey awoke upon Riku's initial touch, he could do nothing until he was thrown to the ground. With his unbroken arm, he pushed himself up into a flip and grabbed the Keyblade in mid-air. He landed in perfect grace on the edge of the bubbling dark pool that spawned the Heartless. His large eyes glared at his assailant, and both took battle-ready-stances...
"Too long. Too long have I waited for this day to come. My power is finally enough to take you on and come out the victor. You might think the darkness has taken control of me again, but you're wrong. I control the darkness! I was locked in here because they were afraid. They had every right to be, too. So why are you in here, old mouse? What did you screw up on to deserve this half-life?"
Mickey's determined face drooped a slight bit at the mention of his hidden truth. His lips unfolded into a snarl, which failed to faze his rival.
"Ha. Too scared to speak, aren't you? I knew it. Nothing but a fool mouse whose 'pure heart' allowed you to wield my weapon! You act all tough in front of everyone, but I know the truth. You're a coward. Then again, I wouldn't blame you if you were afraid. You should be. So, are you gonna try and stop me, or will you be a good rodent and hand over the key?"
Mickey threw the Keyblade in the air, caught it, and pointed it at Riku. "I knew I shouldn't have trusted you right from the off. Once you are taken by the darkness, it's all you'll ever know! Believe me, there's only one way to stop the evil coursing through your veins, and this isn't it! Just stop and you'll get what you really want!"
Riku's body and thoughts came to a halt as he thought of Kairi, the one person who would unquestionably believe him and protect him through his ordeal. All he really wanted was her. She was the one thing on his mind all the time, and her influence now was too much to handle. "K...Kai...Kairi..." He thought next of Sora, and how the two he once called "friends" were probably doing just fine without him, as if he was the only thing stopping them from being together. In his contemplation, he paid no mind to the mouse scooping up a handful of dark plasma and hurling it straight for him.
The Heartless-spawning liquid burned and tingled upon his skin, and Riku screamed in agony. It had covered most of the left half of his face and a section of the right. His left eye was plastered shut by the plasma. The youthful skin mutated to a black and rubbery-smooth texture, not unlike a Shadow's. He tried desperately to open his left eye, but to no avail. He ran about the cavern, dodging Mickey's one-armed assaults by inches at the most. Mickey grabbed another dripping ball of dark goop and sent it flying Riku's way. He finally opened his left eye, which perceived the attack just in time. He ducked, but some still found its way into his hair. The covered strands transformed from silver to black, and lengthened some as well. His left eye was large and glowed yellow, just as a Heartless' eye would. His vision was not impaired, though he now saw half the world in night vision and half the world as a human would. He ran to the highest point in the cave, crouched, and let out a menacing hiss.
"Bring it."