Disclaimer: I am not the owner of Rise of the Guardians or Phineas and Ferb.
For purposes of this story, A/D will be an abbreviation for alternate dimension. I will use it to try and keep characters separated from each other. Let me know if it gets confusing and I will try to think of something else!
Takes place just after Easter in ROTG, and at the beginning of the Phineas and Ferb movie.
Jack was flying through the sky, holding onto his baby teeth in the pocket of his hoodie. Bunny had looked so upset and angry just moments ago, but Jack was not angry with him for what he said. Jack thought that he was right, that he had messed up. Jack should have just ignored that voice and gone straight back to the Guardians. Instead, he ended up flying straight into Pitch's trap. Now, the Guardians wouldn't even give Jack a chance to explain and he was back where he started, not believed in by children and isolated from other spirits.
Jack really didn't know where he was going, only that he needed to get away. Easter was a mess and Jack only felt worse looking at the shattered eggs in parks across the world. He needed to find a place where there wasn't any evidence of what he had done, a place where he could be alone with his thoughts. The arctic was a good idea, generally isolated since people disliked the cold, but Jack decided against it. Maybe instead of being alone, he could try and distract himself. Fun was the best way to do that, so soon he adjusted his course and began flying for the nearest town.
Danville was the name of the town and Jack smiled as he flew over it, seeing some strange contraptions being used for a game. "That looks like fun. What is it anyway?" he asked himself, kneeling down by the contraption. It was being used as a catapult, but it looked almost like a giant blue platypus. While it wasn't quite the same as snowballs, it definitely looked like it was entertaining. At the same time, Jack noticed a dark shadow arrive, the sign that Pitch was here and going after these children.
"And here I was thinking hope would vanish with Easter. It looks like I still have some work to do." he announced grandly, appearing from the shadows in front of Jack Frost. "Ah, Jack Frost. I thought this might happen." he said sadly, trying to portray sympathy for the boy. In reality, Pitch did relate but he couldn't care less about the opinions of the Guardians. That was Jack's issue, not his, but Pitch could relate to the spirit's desire for belief. That was why they were so alike, why the two of them just had to work together to take down the big four.
Jack held his staff tightly, keeping it pointed at Pitch. This was the man responsible for destroying Easter. If Jack defeated him now, the Guardians might forgive him and take him back. He might actually deserve to have his memories, still hidden away within his pocket for now. "Pitch. What are you doing here?" Jack asked, glaring at the boogieman.
"What I always do, Jack. Spreading fear and darkness, to this small town where it seems unusually absent. That needs to change, and I promise you that it will." he announced, creating a nightmare and having it fly off.
"Leave me alone!" Jack yelled, attacking the creature by sending a blast of ice to destroy the horse. As the ice collided with it, the nightmare exploded in a blast of dark sand and ice, freezing in the air and falling to the ground before shattering in front of them.
"There it is, that beautiful combination I've been looking for this entire time. Cold and dark, they mix quite well, don't you think?" Pitch asked, glancing towards the winter spirit.
"You think I'll work with you? You must be crazier than I thought." Jack commented, sending ice towards the nightmare king. Pitch dodged with ease, still smiling at Jack Frost.
"Oh? You do realize that I am the only one who will ever understand you, right?" Pitch countered, dodging yet another blast of ice.
"You don't understand anything!" Jack yelled, still furious over Pitch's little game earlier. More ice was sent towards the boogieman, this time being blocked with a wave of dark sand. As it froze midair, Jack couldn't help but notice what Pitch was saying. Yes, their powers did mix very well. However, Jack was not willing to follow Pitch and what he stood for. Jack cared about children, unlike the spirit he was fighting now.
"Really? I don't understand what it means to be cast out! To not be believed in! To long for…a family." Pitch said, his voice getting softer as he continued. He had wanted those things for so long, but now he was on the verge of giving up. A family would never happen, not for someone like him who could only bring fear. However, he could achieve one of his goals. He could be believed in, if he managed to bring enough fear to this world. Pitch noticed the effect his words had on Jack, enough to make the boy hesitate and stop his next attack.
Jack was thinking about Pitch's words, tempted to actually give in. He couldn't help but understand Pitch Black, after years of being unseen by the world. It was lonely, almost enough to break Jack, and Pitch had been suffering even longer than the winter spirit. "No, I can't. Working with you would only make people fear me, and that is not what I want." Jack announced, his guard still lowered. Pitch grew angry, something that happened easily for him, and glared at the winter spirit.
"Fine, if you won't work with me, then face the consequences. I'll show you just how strong this fear has made me!" Pitch announced, taking advantage of Jack's weakness. He sent a blast of dark sand towards the boy, blasting him into the air and sending him flying towards a skyscraper nearby.
Within the building, Phineas, Doofenshmirtz, and Ferb had just finished making the other-dimensionater. The portal was open, waiting for all of them to step through. Unseen to them, a certain winter spirit came flying into the room, straight through the portal to the other dimension. Pitch Black soon followed, realizing he had made a small mistake. The winter spirit kept interfering with his efforts, so Pitch needed to make sure he was no longer an issue. That staff of his would be all it took to remove Jack Frost from the game, allowing Pitch to be victorious. Pitch Black watched him fly through a strange portal, almost reminding him of North's globes. Sighing, Pitch followed feeling annoyed at having to go through so much effort to destroy a single winter spirit who claimed to be neutral in this game. Things were getting tedious, but Pitch Black was determined to come out on top in the end.
Alright, I'm trying to post all of my ideas at once, since I'm running out of steam in terms of inspiration. Credit will be given to anyone who inspires me to keep going on any of these stories, though I will not guarantee a conclusion since I have not planned one out. There is also a poll on my profile to get an idea of which story readers would prefer me to focus on trying to think of an ending for. Please review and let me know what you'd like to see happen in this story!