Demon Slayer: I, the Master of the Demon Slayer Corps, Turn Everyone into Demons

She transmigrates into Demon Slayer and becomes Ubuyashiki Haruya, the younger brother of Kagaya Ubuyashiki, the current head of the Demon Slayer Corps.

As a member of the Ubuyashiki Family, he has been cursed since birth and is doomed to die young.

To fight the curse, Haruya asks Tamayo to turn him into a demon.

Having escaped the curse in his demonic body, Haruya sets his plan in motion, turning the Hashira and Corps members into demons one after another.

As they say, fight magic with magic!

Why should demons get bodies that can regenerate forever, while the Demon Slayer Corps has to fight Muzan Kibutsuji with fragile bodies and almost no room for error?

Haruya: I say a Demon Slayer Corps made up of demons wielding Nichirin Blades might not be such a bad idea!

Muzan: Guys, times have changed!

PS: (The rating just came out, but it'll go up later.)

[Putting Muzan through the wringer + multiple heroines + demon Corps leader + younger brother of Kagaya Ubuyashiki + everyone turns into demons and awakens Blood Demon Arts + the Twelve Kizuki (and Muzan) awaken new Blood Demon Arts + thrilling battles + comedy + transmigration + no System]

(It's action-packed from start to finish, with a brisk pace. Read with confidence, it's definitely a great story!)