Soni is part of Buddhist terminology that means listening to the precious views of a priestly mentor. It is used in Hossen shiki (a debutant ceremony of young priests in which older priests answer questions raised by young priests). It is also the name of a Buddhist monk.
As he became religious when he saw a picture of hell at Kichiden-ji Temple located at the east side of Kamo-gawa River (the Yodo-gawa River system), he studied the doctrine of Tendai Sect initially under Kenitsu of Kozan-ji Temple and later under Zozen of Gokuraku-ji Temple on Mt. Hiei. Further, he learned the doctrine of Esoteric Buddhism from Gensho and was granted Bosatsukai (Bodhisattva Precepts) by Enchin. He was appointed to the head priest of Tendai Sect in 926. He was known as an ascetic Buddhist monk who was good at such rites like Buccho Sonsho ho and Fudo ho and it has been passed down that his rites conducted during Taira no Masakado's War (Johei War) had a miraculous effect. He was eventually promoted to Daisojo (the highest rank of monk) in his last years. It is said that though he originally aspired to go to the Pure Land after death, he aspired to go to Tusita Heaven, a world where miroku (Maitreya) lives, when he died. After death, he was posthumously conferred the title of Sojo Hoin.