Because it wouldn’t be a traditional action climax without a big twist and a moment when it looks like all hope is lost… here’s all those things I just said.
After a couple episodes more dedicated to talking (and the show stronger for it) this one is mostly action. Kakeru races towards Oka, first being driven by Kippei and then running with his power suit. Our other heroes (since Kakeru’s heroism speeches have, in typical optimistic fiction fashion, been picked up by all his comrades) meanwhile fight off the endless swarms of Oni responding to the ritual. During this period, we see basically all of the incidental characters menaced and saved, either by the timely arrival of one of the empowered cast or else by the ritual reaching its climax that vacuums up all the remaining oni into a vortex of crystals to be sent through the mystic gate. It’s honestly what you kind of want out of CGI action, with some decent choreography (minus the cuts back to Kippei on the run that are a bit stilted) and even funny moments like the guy who had the fried chicken shop in the hero play episode trying to buy off the oni with chicken (and the oni seemingly undecided over whether or not to just take it).
Kakeru bumps into Kaede, Ryo, and the big guy on his way to Oka, and they give him his blessing to use the knowledge he has to save her. Benio is a little more hostile but the words there are brief as Kakeru indicates he’s acting out of love and thus has no room for hesitation.
At the ritual center, though, we get the big twist: The very wormy seeming guy is not, in fact, the evil saboteur. He’s still half-possessed and very down on himself, but he’s not actually the bad guy. Rather, he has a unique awareness of the real evil, a special Shinja called Shuten who was born from the wish of a human to control the power of Oni, and thus has the ability to consume and absorb both humans and Oni. There are no prizes for guessing who this is, as there’s only one character left in the scene, the one always most hostile towards Ibara as a wild card and most eager to see Oka die, her father. He proves more than a match for the first assassination attempt from mister so-snake-like-I-should-have-known-it-was-a-bluff, and Haruka is too much affected by hero syndrome to shoot through him in order to take out Shuten. In a nice turn, she does break free of her own inner oni in the process, recognizing a more noble wish to stand alongside Kaede rather than be her.
This results in Shuten taking charge of the ritual and absorbing all the oni to call his “true body” from the Netherworld, which manifests as, instead of the big meanacing oni we saw eat that oni crystal before (which he was) a pretty cool looking giant with dad’s human body floating near the chest cavity of the monster to grandstand with crazy faces as he gives his typical yet still effective monomaniacal villain speech.
Kakeru (with Benio in tow, the two of them moving out rather than continuing to fight when it was clear something was going pear shaped) shows up at the last second to keep Oka from getting squashed like a bug, but his resistance doesn’t seem to last long, as he’s unable to call Ibara rather than just his white power suit (Shuten claiming to have already consumed him) and is ultimately… well, the animation is “flattened by giant hand” but it’s not like he’s physically damaged so much as absorbed by the monster. This, and the villain speech, hardens Oka’s resolve, causing her to in a literal sense stand up to both the monster and her father and declare that she’ll sever all the bonds of fate if that’s what it takes. Shuten goes ahead and does the squish-absorb on her too, setting us up to have her, Kakeru, and Ibara tear him apart from the inside next episode if the progression is set to be at all sane.
Honestly, this episode was quite standard in a lot of ways, but even if it was cliché after cliché, it was at least the clichés you’d want to see, every good action and drama beat going in time one after another. As to the “surprise villain” I wasn’t really so surprised. I hadn’t thought that Ukon (the guy who seemed so slimy) was going to turn out good, but all the same I’m not really shocked, and especially not surprised that Oka’s dad turned out to be sinister. They set up the general themes, if not the specific details, more than well enough and there’s some degree to which Shikizakura is playing enough into the genre that common conventions like this turn are going to land passing well as it is.