In the MMORPG Cross Reverie, Sakamoto Takuma is so powerful that he is lauded as the "Demon Lord" by other players. One day, he is summoned to a world outside his own - but with the same appearance (Diablo) he had in the game! There, he meets two girls, Rem Galleu and Shela L Greenwood, who both proclaim themselves to be his Summoner. They perform an Enslavement Ritual to turn him into their Summon... but that's when Takuma's passive ability <> activates! Instead, it is the girls who become enslaved! Though Takuma may be the strongest Sorcerer there is, he has no idea how to talk with other people. It is here he makes his choice: to act based on his persona from the game! "Amazing? But of course... I am Diablo, the being known and feared as the Demon Lord!" So begins a tale of adventure with an earth-shakingly powerful Demon Lord (or at least someone who acts like one) taking on another world!
Summary taken from: MyAnimeList