Western Fantasy: I Ate Hero Meat, and It Was Delicious

The monsters ate the hero's flesh and holy sword, and the monster became the Holy Sword Hero. Can he still defeat the Demon King? (Light novel-style characters.) Everyone says stories about heroes fighting Demon Kings are clichéd, but how long has it really been since you last read one? Dual protagonists. There's a map of the other world in the first paragraph of the first chapter's comments, so you can refer to it as you read. The pacing is very slow, so you'll need to take your time.