Zenmetsu End wo Shini Mono Kurui de Kaihi Shita. Party ga Yanda.

Walker, a man reincarnated as an adventurer, is suddenly attacked in a dungeon by a high-level monster and realizes he's in the dark fantasy world of a manga he read in his previous life. Not only that, but he also isn't the protagonist—he's an ill-fated side character meant to die alongside his party to establish the setting's grim tone! Walker fights tooth and nail for his friends, and although he loses a leg and an eye in the process, he manages to prevent everyone's terrible fates. But as he adjusts to a new life with an injured body, his female comrades begin to act strangely toward him!

(Source: Yen Press)