Strike the Blood

The fourth primogenitor - is the strongest vampire who exists only in the world of legends; yet when this masters the 12 kenju, the spread of the catastrophe of the vampires will appear in Japan.

In order to spy on and erase this, "fourth primogenitor," the government owned Lion organization have decided to send an attacking magician, "sword shaman" to complete the task. Yet unknowingly, the one who was selected to spy was actually a trainee "sword shaman" girl - Himeragi Yukina.

The following links are to AnimeNewsNetwork for the anime:
Anime
OAV
OAV II (sequel to anime)
OAV III (sequel to OAV II)