With Shinobu missing and the instinct danger tormenting them liable to show up again at any time, Koyomi and company have finally managed to track down Izuko Gaen in search of help with their current quandary... or perhaps I should say that Gaen has tracked them down.
Given that she claims to know everything, it seems that nobody is better placed to inform Araragi et al how to defeat their current opponent, but before that Gaen wants three promises out of Koyomi - to be introduced to Kanbaru by him, and for both him and Kanbaru to help Ononoki with the work she was tasked to do before being distracted by current circumstances.
Having agreed to all of this, Gaen's answer to Araragi's pleas aren't so much disappointing as downright distressing - although she confirms that not only is Shinobu safe but that she's actually not the true target of this "darkness", the real truth is perhaps even worse. In other words, it's actually Hachikuji who is the target of this darkness, as it seems to restore balance to the unnatural situation in which Mayoi now resides - essentially, she should have passed on to the afterlife by now having achieved her goal and found her way thanks to Koyomi, but rather than doing this or continuing her "work" as an apparition she's instead found herself unable to separate herself from Araragi. As she's adamant that she won't return to a life of ensuring that others get lost, there's nothing for it but for her to accept her fate on move on.
Cue half an episode designed to successfully reduce me to a mess of tears via an elongated but poignant and occasionally rather beautiful farewell to Hachikuji, which adds another hugely memorable moment to the increasing list of such memories provided by this franchise as a whole. The ability of this series to shift from the silly to the serious, and from tears of laughter to those of sorrow, is something which has really come into its own within Monogatari Second Season, and the joy of watching it is the feeling that you never quite know what you're going to get next, even when that something leaves you blubbing. Every time I worry that this show might be dipping into a rut, it somehow manages to surprise me, and long may that continue.