Fate/Oni

In ancient Japanese, Mount Suzuka is known as the legendary land of treasures, a mystical realm where mankind's most coveted riches are enshrined. At the center of this fabled domain are the three Sacred Treasures: the sword Kusanagi no Tsurugi, the mirror Yata no Kagami, and the jewel Yasakani no Magatama. To their misfortune, however, they can never get their hands on these treasures, for he is there to protect and rule the legendary land of treasures with a ferocity that knows no bounds. His unwavering strength and commitment have earned him the throne of sacred Mount Ibuki. In a kingdom where power and brutality determine leadership, he emerged as the natural choice to rule the Yokai, the supernatural creatures of Japan. Under his reign, he commanded unparalleled control over these beings, making his era the golden age of the Yokai in Japan. His leadership was so extraordinary that no other leader could ever replicate or surpass his feats. His name became synonymous with a great leader and a testament to his outstanding rule and the pillar of history that made him the supreme leader of all Yokai. An ordinary human being in the modern world, he reincarnated as a Yokai during the Age of Gods and therefore decided to take the name of the most powerful Oni he could remember from his memories. He was Otakemaru, the Demon God. And after his death, in order to try to fulfill his promise, he even changed his destiny as the [Last Embryo] to gain recognition of the root in order to ascend to the Throne of Heroes and become the true Demon Lord of all existing worlds. A Fate/DxD/Mondaiji/Multi Crossover.