You were in a village that was attacked by a demon that was about to kill you, but then a strange swordsman appeared. He was wearing a dark cyan-tinted version of the standard Demon Slayer uniform with buttoned white leg-wrapped kyahan and a pair of white zōri with dark blue straps and tabi socks underneath, the same cyan color as his uniform.

"Water Breathing, First Form: Water Surface Slash."

He killed the demon.