As peace returns to the demon realm, Anos Voldigoad wishes nothing more than to put his reputation as the Demon King of Tyranny to rest and go back to being a misfit at the prestigious Demon King Academy. Unfortunately, any tranquility is fleeting: sinister demons, kings, and deities plot Anos's demise from the shadows. Rumors spread about the "Child of God," a being whose power may rival that of Anos. To uncover the truth and eliminate the potential threat, Anos must journey deep into the land of spirits. However, the spirit world is shrouded in mystery, and it may only be entered after undergoing a series of difficult trials. With unrivaled power and confidence, Anos braces himself to defeat various formidable enemies with grandiose titles. But he—with the assistance of his trusted allies—will barely have to break a sweat as the true Demon King of Tyranny.

The story centers on Minato Kiyoumizu, a boy who quit playing water polo after an incident during the winter of his third year of junior high school. After entering high school, he makes a promise and decides to try water polo again and join the school's team with his friends. However, the new team will run into problems along the way... (Source: MAL News)

OVA bundled with the special edition of the manga's side story.

In the series' backstory, 102,000 years ago in a place that would be modern-day Mexico, it started with a mysterious but powerful race with incredibly long lifespans and the ability to absorb life forces from plants and animals, although they had the disadvantage of being easily disintegrated if exposed to sunlight and thus lived underground instead. Kars, an underground-dwelling humanoid being known as a Pillar Man, created a stone mask which grants immortality by absorbing life energy to conquer the sun (which kills his kind). However, the stone mask cannot penetrate his head and unlock Kars' true potential. As he tirelessly slaughtered things for blood to activate the stone mask, his tribe begins to fear Kars for his obsession. When his tribe says Kars must be killed, Kars is angered and questions them if they want to conquer the sun. If they objected, Kars would say they will die. As his tribe attempts to kill him, Kars and his companion Esidisi fight back. He slaughtered all but four of the tribe.