Kenseijo Adel no Yarinaoshi: Kako ni Modotta Saikyou Kensei, Hime wo Sukuu Tame ni Seijo to Naru

Blind swordmaster Adel personally put an end to the Great War engulfing the continent in chaos. Though he uses it for good, his strength comes from his horrific past—the gruesome experiments performed on him and the countless battles to the death to prove his worth. It was the gentle compassion of Princess Euphinia that saved him, and Adel meant it fiercely when he swore to be by her side forever. So now, even with the war won, he bitterly rues his failure to protect her when it mattered most. Adel would give anything for a second chance to make things right, and he's in luck. A mysterious being is about to offer him one wish, and he knows exactly what to ask for. He instantly finds himself transported to the past...as a beautiful woman! Prepared to make use of both his old abilities as a swordmaster and his newfound power as a Saint, Adel now leaps into action to change the course of history and save his beloved Princess Euphinia!

(Source: J-Novel Club)