In the very first ages before man, were the Old Ones. Some worshiped them like gods, and they could as well have been- their power was immense. Magic was still thriving, and with that, so were they.
As man began to take over with technology, the presence of magic began to die out. Deities were retiring, and moving away from their contact with humanity. The Old Ones had it no better...
Stories about them passed down began to become scarce, and knowledge that they ever existed began to fade. One of the Old Ones took it upon Themself to make them known: the Bringer of Chaos and the Spirit of Decay. But the humans of this age had tricks up their sleeve, and the remaining with knowledge of magic sealed the great Old One into a stone monolith, where They resided for millions of years, a ghost of the past.
One regular day of 2022, the new age of man, a marine biologist ventured far into the depths of the ocean only to discover an old, strange stone with carvings unlike anything he had ever seen. Curious as humans are, he touched the monolith. The Spirit of Decay washed over the junior scientist like a tidal wave- and a transformation began. The man's body twisted and contorted into shapes and horrors that human eyes would not even begin to comprehend… But at the end of it, the Old One and the human man were one. Bones jutting out of skin where bones should not ever be, but the body was ruined. There along the ocean floor were two halves of the head separated from the biologist's body, as well as varied limbs separated from the torso. This was nothing to the higher being now inhabiting this body; however, as with the help of magic that was released from the stone and the Old One’s experience with the body, They patched Themself back together again.
On the arrival to land many things had changed, and Their power was mostly gone. Keeping Themself anonymous, They built Their own small dwelling in a part of Canada where the marine biologist housed himself. Gathering information of humans of this age was fascinating to Them, and with the current situation They integrated well beyond Their own assumptions. Choosing a name was the easiest. The Deep Sea. The Abyss. Oh, the irony. And thus, Shinkai was created.