Shinobumonogatari

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! The curtain opens on Monogatari Series: Monster Season!

(Source: Kodansha, translated)