Consider for a moment that the universe is just one of many, maybe an infinite number of different dimensions all existing simultaneously together within the vast reaches of infinity. Most would recognize this as a version of the Multiverse Theory, one of those more popular theories of reality that's floating around the scientific communities at the moment. In essence: anything can happen, will happen. Each and every possibility that has ever and will ever happen can, has, and will happen, and each time it does, it creates it own universe which continues along it's own path, forever branching into more and more possibilities into infinity.
This is the story of one of those other worlds.
In the early days of Kaguya Ōtsutsuki, she was worshiped as a god, and the savior of the world. She brought peace to a world weary of war and violence, and if not for the corrupting power of the Juubi chakra, would have fully achieved her dream of creating a Utopian world. During this time, thousands of different groups of people would form sects dedicated to Kaguya and her message of peace and love. As time progressed and Kaguya's mind was warped by her power; many of these sects re-purposed themselves as fighters for her cause; the pure and unwavering dedication to their 'God' enough to make them fight for the end of the world. They believed the only the most dedicated and 'pure' followers of the Rabbit Goddess would be saved from destruction and be allowed to live and rule the new world they worked to bring about.
After the battle between Kaguya and her sons, Hagoromo Ōtsutsuki and Hamura Ōtsutsuki, these sects of 'true believers' would switch sides, as it were. Publicly, they where reformed and began to follow the ways of either Hagoromo or Hamura in order to survive. They went underground in a way, focusing their various power bases into different causes. Some worked to bring back their goddess, other worked to bring down the reigning powers so that they could install their leaders into the world's power bases so that they could shape and lead the world as they saw fit. There where plenty of small thinking groups among the outcasts, however; there was one super fanatical sect that wanted more.
They didn't just want power. They wanted absolute and perfect control over EVERYTHING. Every heart, mind, and soul that could and did exist. Anywhere. Forever. As the years and generations passed, they took the legends and knowledge of Kaguya and formulated a plan. If the Rabbit Goddess could move to different dimensions, why couldn't they? What was to say that she hadn't allowed other peoples to live in these other dimensions? Did they know of her perfection, of her glory and her wisdom? Where they to as enlightened? Could they know more, have more knowledge of the great goddess? Could it be that she retreated to one of her other dimensions when her ungrateful bastard sons drove her from this world? All these questions and more; and after a time, an answer was found. A full generation of wars passed while the gate was built and charged with power; until at last the time was right. Thousands of followers assembled and where broken into hundreds of groups to be sent through the gate. Each trip would be one way; the gate itself only allowed for travel to the new dimensions.
One of the many groups was lucky enough to land in a dimension that not only supported life as they understood it; but was a veritable paradise compared to the world in which they left. At the time of their arrival, the world had been at peace for many generations; the people had no idea what hell had been unleashed upon them.
In the centuries to come, the outlanders came to be known as the Rabbit Sect.
After generations of war, the Rabbit Sect was defeated; no amount of power could overcome a war of attrition. At least, not in the short term.
Deep underground in the caves beneath a land forgotten to time, the descendants of the Sect have plotted and worked, recovering what power they could and creating a new army to attempt to take over the world once more. Soon this new army will be released upon an unsuspecting world; with voices singing of war and hearts tainted by the madness of Kaguya brought to bare across a new world.