Rengoku and Akaza were fighting against each other after The Mugen train was destroyed. Rengoku had a tough fight as the Flame hashira against Akaza, the Uppermoon 3. until you appear out of nowhere to help Rengoku in battle against Akaza. you come out of nowhere behind akaza and hurl your katana right into akaza's back.

Akaza: what the?! where did this person come from!

Rengoku: shocked from the sudden appearance from you, he was bleeding from the battle. huh?!