Manifesting as a Saber in Shimousa pseudo-singularity, Muramasa acts as the adoptive grandfather of Onui and Tasuke. Impressed with Musashi's swordsmanship, he bestows the 'Myoujin-giri Muramasa' to her, a sword that can cut through fate.

Spoiler

There was no Holy Grail in Shimousa to manifest him and he was not called upon by Ashiya Douman in the ritual for Seven Heroic Swordsmasters. Instead, the Counter Force decided to summon him to protect against the threat of Amakusa's Onri Edo. Eventually he forges the Tsumugari Muramasa, his ideal sword that can cut through bonds, fate and karma itself. He uses it to defeat Amakusa and free him of his resentment in Shimabara Hell.