He never asked for this power.
 
Kojou Akatsuki used to be an ordinary high school boy--but that wasbefore he became the Fourth Primogenitor, the most powerful vampire of all time. Worse--he doesn't even remember how it happened. Middle schooler Yukina Himeragi, meanwhile, is the Attack Mage sent to observe him--and to stop him if he gets out of control. But was it really a good idea to send a cute girl to observe a massively powerful vampire boy whose abilities activate whenever he gets aroused? And on an artificial island populated with all manner of demonic individuals, each with their own designs on the Fourth Primogenitor--?!
 
(Source: Yen Press)

Kojou Akatsuki lives on the artificial island of Itogami, and he has a secret: He's the "Fourth Primogenitor," the most powerful vampire in the world. He's carefully kept this dark knowledge from his family and friends, but one day, a beautiful girl named Yukina Himeragi appears before him and tells him that she knows who he really is--and that she's been sent to...kill him?!
 
(Source: Yen Press)

The second season of Strike the Blood. The adaptation of 9th, 11th and 12th LN volumes.

The Fourth Primogenitor—That's the world's strongest vampire that should only exist in legends. Accompanied by twelve Kenjuu and spreading calamity, this phantom vampire appeared in Japan. For the observation and obliteration of this Fourth Primogenitor, the government and Lion King decided to dispatch an attack mage known as a Sword Shaman. However, for some reason, the one chosen for the observation was a apprentice Sword Shaman girl, Himegari Yukina. Wielding the strongest anti-primogenitor spirit spear, Yukina arrived at Demon District, Itogami City. What is the true identity of the Fourth Primogenitor, Akatsuki Kojou, she encountered over there!? 
(Source: Baka-Tsuki, edited)

The third season of Strike the Blood. 
 
It was announced at a Dengeki Game Festival stage event that the Strike the Blood light novel series will get a third OVA release. It will cover until the end of Seisen-hen. 
 
(Source: MAL news)

The fourth season of Strike the Blood.

While several of the girls investigate Vatlers ship afloat, Akatsuki and Himeragi wake up alone in the school gym with no memories of the day before and no explanation for why they were asleep on the same futon and why Himeragi is wearing an old-fashioned school PE uniform with bloomers. A bloody baseball bat and missing weapon soon deepen the mystery as assorted forces race to solve the mystery of the empty boat, the bloody bat, and the purple bra in the MCs pockets!

The spinoff series follows Yukina, Nagisa, and Kanon in their everyday lives in middle school

The fifth season of Strike the Blood.

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