On the ninth day of the tenth month, 1271, on the eve of his departure for Sado Island, Nichiren Daishonin wrote this letter to Chikugo-bō Nichirō, who was later to become one of his six senior disciples. Nichirō and his father had converted to the Daishonin’s teachings in 1254, and Nichirō had received the tonsure under the supervision of his uncle Nisshō, also later to become one of the six senior disciples. At the time of the Tatsunokuchi Persecution in the ninth month of 1271, Nichirō and several other priest-disciples were imprisoned in the custody of Yadoya Mitsunori, an official of the Kamakura government, in a dungeon carved into the side of a hill near Mitsunori’s residence in Kamakura. 

In the interval between the Tatsunokuchi Persecution and his exile to Sado Island, the Daishonin was held at the mainland residence of Homma Rokurō Saemon, the deputy constable of Sado, in Echi of Sagami Province. It was here that the Daishonin wrote this letter, which demonstrates that, even when about to embark for a destination sure to be marked by privation and hardship, his chief concern was for his disciples.

https://www.nichirenlibrary.org/en/wnd-1/Content/26