Yuusha Party wo Tsuihou sareta node, Maou wo Torikaeshi ga Tsukanai hodo Tsuyoku Sodatetemita勇者パーティーを追放されたので、魔王を取り返しがつかないほど強く育ててみた; I Was Exiled from the Heroes' Party So I Tried Raising the Demon Lord to Be Unbelievably Strong

One day, Raizel was suddenly summoned to another world to defeat the Demon Lordbut he was summoned with zero combat power. However, that was just a mistake of the combat capability calculator and he was in fact an overwhelmingly strong individual. After being exiled, Raizel set out on his own and during his adventures came to realize that the demons weren't actually evil entities and joined up with them. Upon learning information which could revolutionize the state of the world as it currently stands, Raizel and the demonkin companions set out to take down the empire that had summoned him to this world. (Source: Futabasha, translated)