Hōraijima

was a place where yōkai and humans lived together in peace, resulting in many hanyō children. Since hanyō were unaccepted by the world, a barrier was put up to separate them from those who would shun them. Every fifty years, the barrier would weaken, and other yōkai would attack the island only out of disgust for the existence of the hanyō children. Among them, were the Four War Gods. There was a battle, and to stop it, Lady Kanade, a priestess from the island, sealed the Four War God's powers into a jeweled box, and threw the box into the sacred Cauldron of Resonance. When she did this, her spirit was sealed into the Cauldron as well. The War Gods realized, that if they threw a hanyō child into the Cauldron, they could regain their power, little by little, though they were only able to do this every fifty years, when the barrier weakened. When the Cauldron of Resonance was destroyed, the island collapsed. Now, it is nothing but a memory.

Trivia

 * Hōrai, or  as it is known Chinese mythology, was a rumoured mystical land with two different origin stories. The movie depicts it based on Japanese mythology.

Horai-Island Isla Horai