Starting with the Sharingan: Infinite Death-Seeking in Demon Slayer

Tetsuya Kamihara thought his luck couldn't get any worse after being killed by a runaway truck, but being transmigrated into the flesh-eating world of Demon Slayer quickly proves him wrong. To make matters infinitely worse, he opens his eyes on day one only to find Upper Rank Three, Akaza, standing right outside his door. Just as he is about to be slaughtered, he awakens the "Unlimited Death-Seeking System." The rules are brutally simple: the closer he dances to the edge of death, the greater his rewards. His very first mission? Provoke Akaza. Betting his life on a sliver of hope, Tetsuya secures the legendary Sharingan from his system and convinces Akaza to turn him into a demon using Muzan Kibutsuji's blood. Now a demon who refuses to eat humans, Tetsuya begins a high-stakes tightrope walk between two warring factions. Using his new demonic resilience and his system-granted abilities, he manages to save the Flower Hashira, Kanae Kocho, from her fated death. In a bold and borderline suicidal move, he infiltrates the very heart of his natural enemies—the Demon Slayer Corps. By day, he hides from the sun, working as a brilliant medical prodigy at the Butterfly Mansion under the noses of the Hashira. By night, adopting the enigmatic alter-ego "Yasuo," he uses his Sharingan to copy Breathing Styles and clashes with the Twelve Kizuki. With his system constantly pushing him to provoke the most terrifying beings in existence—from Muzan himself to the strongest Hashira—Tetsuya must lie, cheat, and court death at every turn. After all, the higher the risk, the richer the reward! — Description from novelcodex.org