Strike the Blood, Vol. 2 (light novel): From the Warlord's Empire, Vol. 2 , (Novel) The Right Arm of the Saint

The tentative peace between Kojo Akatsuki, the Fourth Primogenitor, and Yukina Himeragi, the girl sent to observe him, ends with the appearance of Dimitrie Vatler, an emissary of the Lost Warlord.
Kojo Akatsuki--the Fourth Primogenitor--is finally starting to get used to his new life. This includes getting used to the presence of Yukina Himeragi, the girl sent to observe him. But this tentative peace is brought to an end by the appearance of Dimitrie Vatler, an emissary of another primogenitor--the Lost Warlord! Along with Dimitrie comes his observer, Sayaka Kirasaka. Dimitrie has a strange fascination with Kojo, but Sayaka just wants to kill him! Their arrival, though, is a mere prelude to the shadow that soon falls over Itogami Island, threatening its very existence!