Traditionally born 1435, died 1522. Based at Nakano Castle in Shimosa, reportedly a guest retainer of the Hara clan. In 1487 he attacked the Hatakeyama clan at Toke Castle, moving his base there in 1488 and founding the Toke-Sakai line. A devout Nichiren-Hokke follower, he is said to have founded Honjo-ji Temple on Nakano Castle's former site. However, clan genealogies name "Sakai Seiden" as the true founder, so some scholars treat Sadataka as an alternate name for Seiden or a later fabrication — his historicity is debated.

Tradition credits him with the drive to rise from guest retainer to independent lord, and strong religious conviction in using Nichiren-Hokke Buddhism as a tool of governance.

Honjo-ji Temple, founded by Nittai, stands on the former castle site, preserving earthworks within its grounds. His descendants split into the Toke-Sakai and Togane-Sakai lines, both prominent in northern Kazusa.