Setsuna punches Muzan's arm, and this time it eventually splits making Inosuke fall to the floor, too weak enough to get up. Muzan groaned and faces Setsuna. Her face was bowed down and at that moment, she looked at Muzan. Kotoha, now returned with several other Demon Slayers, gasped.
"That's impossible! There's no way... Setsuna... she's related to the ancestor of the Spirit of Dawn!" She exclaimed.
The crystal dagger perched itself around Setsuna's neck. Its magic even allowed her hair to go down and add red streaks; she glowed a yellow colour that outlines her body, her eyes closed. Muzan stepped back in fear, taken aback by the power. In a split second, the sun was dawning in the early hours of the morning making Muzan growl. He regrows his arm back and leaps forward to attack Setsuna. But Setsuna had other ideas.
Slowly and carefully, she opens her eyes the moment the sun came up, the necklace began to glow brighter than ever. Setsuna's hairband disintegrates letting her hair loose from the howling wind.
Muzan tries his best to block the sunlight away from him, but Setsuna takes one step towards him and says:
"Flower Breathing, Final Form... Spirit of Dawn!"
Without hesitation, Setsuna releases a burst of light on Muzan, yelling with pain. The Demon Slayers and the demons stopped and saw the beaming light that glowed on the top of Eden Castle so much, they had to cover their eyes. As Setsuna forced every last bit of sunlight onto the monster, Muzan's spider-like demon form begins to ignite and injure him from the inside causing him to flinch from the pain as he begins disintegrating from the sun's light. As his body fully burns into ash, the once invincible Demon Lord who ruled the shadows of Japan, spreading misery and suffering for millennia, finally laid in defeat and fades into oblivion.
As the Spirit of Dawn's light starts to fade away and the crystal dagger stops glowing, Setsuna collapses to the ground and faints from exhaustion. She defeated Muzan which also allowed the rest of his demon army to fade into oblivion, too. The last thing she remembers was people carrying her and seeing Inosuke's body being picked by one of the Demon Slayers, the conversations were inaudible. Her mind went blank and her eyes began to close.