Strike the Blood, Vol. 4 (manga‪)‬

At the invitation of Dimitrie Vattler, emissary of the First Primogenitor, Kojou and Yukina attend a yacht party. Dimitrie's guardian has it in for Kojou, and as though that's not bad enough, the festivities turn serious when Kojou and Yukina discover a terrorist plot that threatens Itogami Island itself...