Taira no Tsunemasa, the son of Tsunemori, was the deputy of the grand empress's household. He was ordered to left the capital with other members of the Taira clan. He once severed the prince-abbot of Ninnaji, so when he was forced to flee the capital, he first visited his teacher, the prince-abbot, in order to leave Seizan in good hands. Seizan (meaning “green mountain”) was named after the painting on the front of the instrument.
The story of Tsunemasa's flight is followed by an episode describing Seizan's origins. Brought originally from China as a gift for the imperial family of Japan, it is a beloved instrument of Emperor Murakami 村上 (926-967). Once while he was playing Seizan, he was visited by a ghost who uses Seizan to teach him a secret piece. From the imperial family, Seizan is entrusted to the Ninnaji Abbot, who gives it to Tsunemasa in recognition for his great talent when Tsunemasa is sent to Usa shrine as an imperial messenger. In the Engyōbon variant text, Tsunemasa’s performance on the biwa inspires the deity of Usa to manifest itself as a dragon above the shindan; it is remarked in all versions that the monks of Usa “could not have mistaken the sound for that of a passing shower (murasame to ha magawaji mono wo).”[2] This rendering is probably the basis for the description of the visit to Chikubushima in the recited-line texts like the Kakuichi-bon. All of these episodes and their variants are later incorporated into a noh play entitled Tsunemasa, in which the ghost of Tsunemasa returns to Ninnaji during a service dedicated to memorializing him after his battlefield death. The ghost takes up Seizan and plays it, then returns to the netherworld. A recording of a performance by the Kongo school of noh can be found here: https://www.youtube.com/watch?v=fr0atOxee6I.
[1] . There are setsuwa about storied biwa in Jikkinsho and Kojidan describing Seizan that seem to be the origin for the Heike account. Tomikura, p. 420 discusses this relationship, as does Yamashita, p. 305.
[2] Kajihara and Yamashita, p. 53.
Below: Tsunemasa and his Biwa