Zaraki Kenpachi and Nnoitra were fighting, they got bounced across dimensions somehow (mix of all the high-powered people in one area, the power clashing, and then Orihime's powers being thrown into the mix, maybe?), and didn't really care to
stop fighting. Because you know Kenpachi wouldn't just stop fighting, when he has someone around that isn't a weakling... and Nnoitra just wouldn't care.
The ninjas tried to interfere, because they were carelessly destroying part of Konoha, only to get bitchslapped in the process. Unable to stop them, the Yondaime prepared to seal them.
So the Yondaime called on
their Death God.
At this point there are two potential variations. Either one of them finally wins, and only one of them gets sealed. In which case, Naruto either has Kenpachi or Nnoitra sealed into him. Personally, I'd rather see the Kenpachi route.
The other option is that they're still fighting, and so he has to seal them both. In which case Naruto gets Kenpachi and Nnoitra gets sealed into Itachi, Sasuke, or Sakura (Nnoitra sealed into a girl? He'd be
so pissed!).
Personally, I think the idea of them being sealed into Naruto and either Sasuke or Sakura has the most potential to be interesting. And if Nnoitra was sealed into one of the Uchiha, how would his Hole being in one of his eyes effect that eye of the Sharingan user? Just something to think about.
Or for sheer fun, Nel got bounced along, too. Kenpachi in Naruto, Nel in Sakura or Sasuke, and Nnoitra in Sasuke or Sakura. Would whoever got Nel chibify whenever they weren't using their powers? Would Sakura and Sasuke develop a
dislike for each other? Either way, that'd likely be one scary team.