Disclaimer: I do not own Ben 10 or Code Lyoko.
This story starts between Bragging Rights and Dog Day Afternoon in the CL universe, and between the events of the original Ben 10 series and Ben 10: Omniverse, while Ben is still ten years old. Keep in mind some chapters of this story have been rewritten and slightly expanded due to my past self being a terrible writer.

Code Omnitrix

The Void

"Almost… got it… and… done!"

Grandpa Max, Gwen, and Ben had been enjoying a quiet break on the beach in between alien attacks. Max had been using the time to add a new attachment to the null void projector. While he was working, Gwen and Ben were off challenging each other, with Gwen doing a hurricane spell while Ben as Stinkfly tried to stay steady in it. As soon as Max said he was done, Gwen stopped the spell and ran over to the rustbucket to see.

"Cool, grandpa!" said Gwen looking at the new attachment. After landing and transforming back, Ben came over to see what Gwen was so excited about.

"What's so special about a- WOAH grandpa, That thing looks so much cooler now!" The new projector was now in rifle form, complete with a scope.

"Well, the only thing left to do is give it a field test," said Grandpa Max. "I wonder where we could find something-"

All of a sudden, a meteor hit just to the left of them, and it stood up, obviously one of Vilgax's robots.

"Well, that was convenient," Gwen said.

"It probably won't enter the null void portal without a little encouragement," said Max.

"Well, I think I can give it some!" Within moments Diamondhead was standing right where Ben was before. "You stand over there and project the portal, while I go 'distract' it."

Gwen, as usual, was fed up with Ben's apparent superiority complex. "Whatever, Ben, just go attack it."

"My pleasure."

Against Diamondhead, the robot never really stood a chance; he had it pinned in just a few minutes. Max, at a safe distance, sniped the portal right behind the robot. Ben promptly threw the robot into the portal and gave the thumbs up to say it was ready to be closed.

However, when Max tried to shut it down, the portal lingered. It flickered a bit, and then collapsed into a smaller blue portal, instead of the usual yellow. "Grandpa, what's going on?" asked Gwen.

Max replied, "When I reconfigured the focusing lens, I think I must have gotten the scale a bit off. The first portal almost definitely led to the null void, but there's no telling which dimension this one leads to. Ben! Get away from there!"

But it was already too late. The Omnitrix symbol on Diamondhead's chest began to blink red rapidly, and timed out before he could move. He turned back to Ben, and without the extra weight of the crystal alien he just was, was sucked into the new blue portal. Max and Gwen tried to get over in time, but the portal closed before they could get there. Ben was now lost to an alternate reality, and there was no way of knowing where he went.

"Is there any way we can get him back?"

"We could try firing it again and going in after him, although I don't know if it'll take us to the same place."

"We have to try!"

"Of course. Come on Gwen, let's go." Max fired the NVP again, and when it collapsed into the blue portal, they started their wild goose chase through the multiverse.