THE STRONGEST HERO: ENVOY OF DARKNESS -BETRAYED BY HIS COMRADES, THE STRONGEST HERO JOINS FORCES WITH THE STRONGEST MONSTER-

There are always 16 heroes. Amongst them the Darkness Hero, Shion, the strongest hero in history. Shion has only one thing he believes in, his party members. However, those girls he believed in cruelly betrayed him. Is there a way for Shion to overcome his deep pain and keep on living?(Source: Manga UP!) — Description from AniList community (anilist.co).