I’m Determined to Save Humanity, So Why Is the System Turning Me into a Ghost?

(Demon Slayer Fanfiction)

After transmigrating, Fushimi Nagamasa arrived in a world where demons and humans coexist, and vowed to slay demons and prove his path to the Dao.

But after absorbing Muzan's blood...

“Ding! System update successful. Main quest unlocked: Become the next Demon King!”

Fushimi Nagamasa: ???