Strike the Blood IV

Kojou Akatsuki wakes on the shore of Onrai Island remembering nothing except his own name. His memories are gone, and he cannot summon the familiars that prove he is the Fourth Primogenitor, the world's most powerful vampire. A white-haired girl named Shizuri Kasugaya Castiella appears before him and introduces herself as his new observer. Onrai is an isolated Demon Sanctuary far out at sea, and away from Yukina and the companions he normally relies on, Kojou is drawn into a dangerous mission alongside people he does not know whether he can trust. To regain his power and return to his life, he first has to discover what happened to his memories and why he awakened on this particular island.