From the Start, I Have the Mangekyo Sharingan, and I Enter Demon Slayer!

The protagonist wakes up as Uchiha Chuan and, by a twist of fate, enters the world of Demon Slayer.

Ding-dong! Congratulations, Host, you've earned the Clan Annihilation achievement. Reward: a pair of Mangekyo Sharingan.

“Since you demons don't play fair, don't blame me for showing no mercy.” Uchiha Chuan gives a sinister smile, and the coffin before him bursts open with a bang.

“What do you think life is?!” cries the man in a bright red robe, swinging his blade.

“I admit that humans have greater potential than demons.” The proud Akaza lowers his head.

“Please, I won't eat people anymore...” For the first time in his life, Doma feels the clear threat of death.

“Thank you for helping me understand my younger brother's feelings.” Tears glimmer in Tsugikuni Michikatsu's six eyes.

“Who are you?!” Kibutsuji Muzan snarls at Uchiha Chuan.

“I... am naturally the one who controls everything, the chosen master of fate!”