High School DxD, Vol. 16 (light novel)

Issei and the others enter the country of vampires and meet Gasper´s old friend Valerie...who now rules as queen! But it´s clear the real power behind the throne is her elder brother, and he´s got her under some kind of mind control, putting her in a dangerous mental state. Gasper vows to save her, but will he prove up to the task?