Tobirama was supposed to have passed on the day that the Kinkaku squad ambushed his team.
In this timeline, he created a sloppy seal to get him back to the village and avoid getting exploded by the dozens of explosive seals he had placed.
After pushing chakra into the seal and activating it, his vision swirled into a dark abyss, only to awake in a world he wasn't aware of. When he came to, he was in some sort of large dome like building with many areas with different natural disasters.(it was an advanced training simulation, but it was so modern to him he didn't realize it was.)
Its not every day you end up in another universe, but like the ice queen Tobirama is, he was barely phased by this("tis but a scratch–").