High School DxD, Vol. 16 (light novel)

SIBLING SHOWDOWN. Issei and the others enter the country of vampires and meet Gasper's old friend Valerie. who now rules as the nation's queen. But it's clear that her older brother is the one pulling the strings behind the throne. Gasper vows to rescue her from her brother's clutches and behead the puppet regime, but it might be too much for him to handle alone. Luckily, he knows a few devils willing to go head-to-head against vampire royalty.