Saeki Koreharu served as the tenth lineal head of the Saeki clan. He also constructed and served as lord of Togamure Castle in the Amabe District of Bungo Province in Kyūshū.
In 1527, Koreharu was falsely accused of colluding with Kikuchi Yoshitake of Higo Province to plan a rebellion against Ōtomo Yoshiaki, causing Yoshiaki to order Usuki Nagakage to subdue Koreharu. Near the end of the year, Nagakage commenced an attack against Togamure Castle, but it was impregnable so Nagakage promised to put in a good word for Koreharu to Yoshiaki provided that Koreharu turn over the castle. Koreharu complied and planned to escape to Hyūga Province. This, however, was a trap, and once Koreharu opened the castle, a local clan of influence known as the Niina launched an attack upon orders of Nagakage that blocked Koreharu from retreating. He then killed himself on Mount Otakachi in Mikawachi.
Koreharu was succeeded by Saeki Koretsune (Koreharu’s nephew – the son of his older brother, Koreyasu) as head of the clan. The familial connections to Saeki Korenori, however, are not clear. The heads of Saeki clan were Koreharu (tenth), Koretsune (eleventh), and Korenori (twelfth).
Nagakage died of illness im 1528, the year following Koreharu’s death. Then, the surrounding area experienced a series of disturbances and calamities. It was rumored that this was the curse of Koreharu, leading to the construction in 1538 of the Tomio Shrine to pacify his angry spirit. The Otakachi Shrine was built on Mount Otakachi to worship his body, the Otō Shrine for his head, and the Tobio Shrine for his katana, or sword, and wakizashi, or short-sword. In total, over twenty shrines were built in the border area between Bungo and Hyūga provinces to worship Koreharu.
The Saeki descended from the Ōga, a famous clan in Bungo dating to the Heian period. The clan believed in snake deities and feared their mysterious powers. Koreharu was referred to as “yato-no-kami” or god-of-the-night sword, snake deities rumored to bring familial extermination on anyone who saw them.