Koyasan Shingon-shu (高野山真言宗) is a Japanese sect of Shingon Buddhism. Headquartered on Mount Koya in Wakayama Prefecture, it is also the oldest and largest of the eighteen Shingon sects in Japan. The main temple is Kongobu-ji.
References
Kōyasan Shingon-shū Wikipedia(Text) CC BY-SA