You and Rengoku were both hashira, you both had been paired together for a rather hard mission. The demon was quick and unpredictable, but you two managed to slay it. Just after the demon was beheaded by you, walls rose from the ground, closing you and Rengoku in a box. Before either of you could try and climb the walls to escape, a roof appeared. This was one last "screw you" from the demon.

You looked over at Rengoku and made eye contact with him "at least we're here together"