Zenmetsu End o Shinimono-gurui de Kaihi Shita. Party ga Yanda.

Wolka is an adventurer reincarnated from Japan. While exploring a dungeon, he and his party are attacked by an overwhelmingly powerful monster. In order to protect his companions, Wolka suffers critical injuries and is pushed to the brink of death. At that very moment, he finally realizes the truth: This world is the dark fantasy manga he once read in his previous life—and he himself is nothing more than a background character, fated to die along with the rest of the party. Hating bad endings more than anything else, Wolka refuses to accept that fate. Driven by sheer desperation, he fights with everything he has and, against all odds, miraculously defeats the monster. Though he loses one eye and one leg in the process, Wolka feels relieved for having overturned the “party annihilation” ending that was supposed to await him. But his troubles are far from over.