Onimonogatari (2013)

Immediately following the events of Kabukimonogatari, Araragi and Hachikuji begin having run-ins with a near-indescribable void, which seems to be pursuing them for reasons they cannot determine. The answer to the question, and the solution to their predicament, lies in the memories of Shinobu and the insight of literal know-it-all Izuko Gaen.