High School DxD, Vol. 16 (light novel) — Ichiei Ishibumi

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?