The dimension of Draconic Deus, otherwise known as Highschool DxD was a very unique world. The world itself was relatively average compared to other worlds as it possessed seven continents and seven seas with many lakes and rivers. What made it unique was that this world was populated by Humans, Devils, Angels, Fallen Angels, Youkais, Dragons, Gods and all sorts of other creatures. Having so many species ensured that Earth possessed a long and brutal history with many wars and battles in it. However, there existed a secret in the modern age that very few remembered. That secret was that there used to be 8 continents in the world.
Roughly 1000 years before the 21st century two beings from outer space came to this world. One of the being's name was Kaguya Otsutsuki and here is where our tale begins. these two beings landed in the eighth continent later be named as the Elemental Continent. Upon landing on the Elemental continent, the two beings quickly made their way towards a very unique tree that was planted there, called the God tree. It was there were Kaguya struck down the other being that was with her. She then lived with the locals for a while before she was forced to eat the fruit of the God tree and awaken her bloodline powers. She possessed many powers, but the most important was called the Rinne-Sharingan. She quickly took over the entire continent and put a barrier over it. This barrier transported the entire continent to a different dimension.
At this point, the Gods of this world, particularly the Shinto Pantheon who possessed believers in this Continent, came knocking on her door to complain. Unfortunately, at this point it was too little done too late as the barrier Kaguya put down was truly too strong. That wasn't to say that it was invincible, it was just anyone who could have destroyed the barrier had no interest in doing so and while the Shinto Pantheon was strong, it was not strong enough to destroy this barrier. That being said, almost everyone paid attention to what went on inside the continent.
They saw how Kaguya had children who went on to inherit her power and seal her away into a massive meteorite. So massive was it in fact that it was frequently mistaken for the moon. They also went on to see how Hagoromo Otsutsuki divided the God beast known as the Ten tails into 9 different divine beasts called the Biju. They also saw how Hagoromo shared the power known as Chakra to humanity and how he and his sons Indra and Ashura created the Shinobi Creed. Everyone stopped watching so closely after Hagoromo died but they still kept tabs on this continent from time to time.
The Shinto pantheon were heartbroken over losing their main home base but they just made another one at a nearby country called Japan. They never managed to actually obtain a foothold on this continent again but due to their still being believers on this continent who prayed to them they had a level of control still remaining. They used their limited powers and created a Pseudo afterlife called the pure lands on the continent and constantly watched over the continent.
They laid witness to the passing of history, the warring clan's era, Hashirama Senju and Madara Uchiha, the creation of the hidden villages and the 4 great Shinobi wars. Our story starts at the conclusion of the Fourth War where Madara and Kaguya were resurrected and subsequently defeated by Naruto Uzumaki and Sasuke Uchiha.