A Wolf in Sheep's Clothing Ao no ekusoshisuto: Kyoto fujô ô-hen : Season 1 Episode 6

Shirou Fujimoto, Rin and Yukio's adopted father, encountered Tatsuma Suguro while trying to take the demonic sword, Kurikara. Tatsuma's letter reveals the circumstances of how the sword, which is the Myoda Sect's main object of worship, came to Rin.