Baroque

Baroque summary:

Kabayama Atsushi was a child prodigy, but now he’s just a regular high school boy. One day, a mysterious man from another world named Trance appears before Atsuhi and asks for his cooperation. Apparently Atsuhi is the chosen person who can save the world—by sleeping with Trance’s “sister,” Tsubasa! Already confused and overwhelmed, Atsushi is also targeted by a woman with strange powers that seems to know Trance and is determined to kill Atsushi. Does the world’s fate really depend upon Atsushi having sex with Tsubasa?