Monogatari Series: Monster Season

"It's chic to go saving a girl who has nothing to do with you every once in a while." Female students from Naoetsu High School have gone missing one after the other. When the mummified bodies of the girls are discovered, there's a very distinctive marking left on the napes of their necks...? Koyomi Araragi, now a first year university student, goes on the move to track down the culprit!

(Source: Kodansha, Bakemonogatari Wikia)