Dohjutsu (瞳術 Dōjutsu) is "The Ninja Eye." Shiunsai concealed it in the pupil of Rikimaru's right eye because he wasn't ready, according to Tatsumaru. Dohjutsu began to glow in Rikimaru's right eye during his fight with Tenrai, allowing him to see when he was blinded.
The true nature or purpose of this power is never fully explained.