In regards to the MMORPG Cross Reverie, Sakamoto Takuma boasted an overwhelming strength that was enough for him to be called the Demon King by the other players. One day, he gets summoned to another world with his appearance in the game. There, there are two people that insist, "I am the real summon master." Takuma gets slave-turning magic used for summoned beasts cast on him by the girls -
However, the peculiar ability Magic Reflection was invoked! The ones that were turned to slaves were the girls! Takuma was bewildered. He's the strongest magician but, he doesn't have any communication skills. Panicking, the first words that came out were the ones that he used inside the game role playing the Demon King!? "I'm amazing you say? Of course I am. I am Diablo... The one feared as the Demon King!"
This is the story the Demon King (acting) that would soon inspire the world and his other world adventure that he plunges through with his absolute strength, raise the curtain!

Unskilled in both swords and bows, Regis is a soldier who ranks last in military academy and is obsessed with books. After being banished to the borders, he met a girl who changed his destiny. With her red hair and crimson eyes, Princess Altina possess the sword of kings. Daughter of a royal consort, she was appointed as the commander of the border regiment at the tender age of 14. But she did not lament her circumstances and bears higher aspiration. "I believe in you." Regis accepted the request of the girl to be her strategist and forge through many hardships. The sword princess and the bibliophile youth embark on their fantasy war story.

Regis Ulrich is an incapable soldier; bad at both sword and archery, who passes the time by only reading books. While transferring to a remote region post, he meets a girl who turns out be the red hair and crimson eyed princess Altina, the wielder of the great sword "Grantneil Catoll." Despite being only fourteen years old, she was appointed to become the commander of the army in this remote region simply because she was the illegitimate child of the Emperor. However, she continues to pursue her dream of becoming Empress despite the circumstances that surround her. To achieve this goal, she asks Regis to become her tactician and stand alongside her against the obstacles that are sure to come.

With the passing of the her grandfather the Eisei Meijin whom she'd spend most of her childhood together, Ayumi lost sight of her goal. On the other side, Erika who's aiming to become a pro at Shogi to pay the dept her father left with them after running away. Right after entering high school the two meet at the on thing they shared in common, the Shogi Club. Find out how their story unfolds as they go through life centered around Shogi!