Monogatari SS: "Shinobu Arc"
I just got fucking wrecked.
I have faith that the Snake Arc will pay off in the final episodes, but I almost wish it didn't exist so that I didn't have to feel like I needed to power through that arc in order to get to this one. Who knew that the innocuous ending to the original Snail Arc from Monogatari ends up being such an important event?
As Corvo mentioned, we also get an explicit reference to Ararararararagi's lack of agency as a "harem" protagonist, effectively pulling apart the character from the inside. That he's called a child by the two "children" in the scene only emphasizes how very much he isn't the harem protagonist at all.
One completely unrelated note is that the idea of the anti-existence reminds me of
KoTOR2 of all things, where Obisidian set up the idea of the "anti-Force" at the end of that game that served as a true counterpoint to the Force. It shifts the paradigm away from Good vs Evil (or Human vs Oddity) into something much more abstract as Existence vs Non-Existence. At the very least, it explains why the Snake arc had to happen, because she was lying all this time, denying her existence.
And in way, although each season has a specific theme, you can see this as a theme that is a central through-line for the entire series - that lying, denying your existence, is a destructive activity. In fact, it's more destructive than any consequences telling the truth might have.
This show is just amazing.