Koen (1074? - July 16, 1169) was a priest of the Tendai sect in the late Heian period. He was born in Higo Province. He belonged to the line of FUJIWARA no Michikane, of the Northern House of the Fujiwara clan (the great-great-grandson of Michikane), and his father was the provisional governor of Mikawa Province, FUJIWARA no Shigekane. His older brother was Shonagon (lesser councilor of state) FUJIWARA no Suketaka. He was called Higo Ajari or Kotokuin Ajari. He was the master of Honen.
He studied under Kokaku, of the Sugyu school in Mt. Hiei, and became a priest, and studied Esoteric Buddhism under Joen. After that, he lived in Toto (eastern section) Nishidani of Enryaku-ji Temple and taught Tendai doctrine; Honen, the founder of the Nihon Jodo sect (Japanese Pure Land sect), studied there, too. Also, he compiled 'Fuso ryakki,' a Buddhist historical book consisting of articles from Emperor Jinmu through Emperor Horikawa, written in classical Chinese and in chronological order. Based on the method called Tendai contemplation, he is said to have conducted various harsh ascetic practices.
There is a legend stating that on July 16, 1169, at the age of 96, he became a giant snake at Sakuraga-ike Pond (Omaezaki City) in Shizuoka Prefecture and went into Bosatsu-gyo practice, wanting to earn the spiritual power to save people. There is an epigraph of Ajari-do Hall standing in Oshokyo-in Temple, which is Oku no in (the inner sanctuary) of Sakuraga-ike Pond, and that is regarded as his tomb.
Further, in Tamana City, Kumamoto Prefecture, which is Koen's birthplace, there is 'Renge-in Tanjo-ji Temple,' which commemorates Koen.