Strongest Sage With The Weakest Crest (Manga) Vol 14

Matthias manages to barely defeat Zaridias, ensuring the safety of the Eis Kingdom, but a new threat arises elsewhere. Something strange is going on with the water in the Syhill Empire and demons seem to be the likely cause. In order to infiltrate the imperial waterworks, Matthias and his party must intentionally poison themselves with a life-draining toxin! Once inside the waterworks, Matthias finds a person from his past life who is somehow alive and inexplicably working with demons! Square Enix Manga
(W) Shinkoshoto (A) Kansho & Hyoko, Huuka Kazabana