Across time, in another reality far similar from this one, the Sage of Six Paths battled against the ten tailed demon, neither realizing the ramifications and ripples that affected the multiverse. The final effort of the Sage resulted in the taming of the demon, a small tear in the universe began to crawl its way between the realities, un-noticed through out millennia. Until the era that the tear has become wide enough for most sensitive beings to become aware of it.
The Second Hokage was one of these and through the cooperative efforts of four other Kage's, they tried to seal off the tear from affecting their own reality, but their efforts only decelerated the breach, allowing beings from other realities to bleed into their universe. An understanding was made between the Kage's of other hidden villages to keep the events of their efforts a secret, that may keep the breach from growing uncontrollably. Sealing the breach as much as they can, they all agreed to keep the secret in hopes of no one dare to rip the tear open.
While the rumors grew about the transpiration, a decree was enforced to prevent the knowledge of this to spread. However, fear still scattered among the people and some used this fear to conspire and use the knowledge for their own gains. Most kept to their own lives as normal, but warily preparing themselves for the worst. More encouraging their youths to train as Shinobi to be able to prepare themselves more against the unknown threat.