SEIJO NO SOSHITSU GA NAI NODE, LAST BOSS WO MEZASU KOTO NI SHIMASHITA

The Divine Law Knights offer up what is precious to God and challenge monsters with the power of "oaths." A young boy named Luca lost everything when his hometown was burned down, and longed to join the Knights. During the oath ceremony, a monster suddenly appeared and attacked Luca, and he was driven to the brink of death without being able to offer anything. At that moment, a sealed, forbidden flame was born within the boy. Will this vow and flame bring hope to the Knights, or will it become a new disaster? A full-scale dark battle fantasy begins as the forbidden flame burns!(Source: Studio Zoon, translated) — Description from AniList community (anilist.co).