Beyond Their Sights
As I was wondering about last week, Yhwach absorbing Mimihagi did indeed have an effect down below, as we see Ukitake collapse and Mimihagi flow out of him completely. Immediately afterwards, as Urahara comments it seems to be being absorbed, the black arrow that Yhwach fired down last week from the Soul King Palace reaches the Seireitei and encompasses it in a dome.
Here's where both Mangastream and Mangapanda messed up last week - better translations (including Viz Manga) have Yhwach say he "will put a lid on the Seireitei", thus ensuring that no one gets in his way as he absorbs the Soul King. So he wasn't "leaving Seireitei be", but ensuring that no one could come from there to interfere. This leaves the Gotei 13 with a big problem, as the Tenshiheiso seems to need a direct path between one location and another in order for the Gate to connect the two places. Otherwise, as it was with the King's Barriers, it will not work.
Unless Urahara is putting up an act and he actually knows what is going on, this shows that even Urahara is possibly out of his depth in dealing with the current situation, which relates to the title of this section of my review.
From what I have observed, one of the keys behind Yhwach's success as a villain is how little the protagonists know about him. Yamamoto believed that he knew all there was to Yhwach's power, and even Squad Zero's intelligence on Yhwach came from Yamamoto's fights with Yhwach. All those signs point towards Yhwach not using "The Almighty", the Auswahlen, or the wider range of his abilities one thousand years ago during the Shinigami-Quincy War. This makes it quite a glaring detail when Isshin expressed knowledge of the Auswahlen whereas the Squad Zero had no knowledge of it (adding further to Urahara likely putting on an act).
On the other hand, not only does Yhwach knows his enemies well, but he knows how to manipulate them, and he is able to use his true ability to full effect in battle as a result of his enemy's lack of knowledge of his power. Even now, when he's effectively won the war, we still don't know the true extent of his plans, and he has yet to utilize Zanka no Tachi for anything, having done all this under his own power and not the power he stole from Yamamoto.
This is not the same as Aizen was in the Arrancar Saga, where we all knew what Aizen's ability was (even if he never bothered to use Bankai in the entire arc), and we all knew what his main plan was. What we still don't know, however, was Aizen's true reason behind wanting to kill the Soul King, as his dialogue at the end of "Deicide" points towards new territory.
With Yhwach, we've only learnt about the nature of all his abilities recently, and we don't know what is going to happen in the following 7 of the "Final Nine Days". Given he's killed the Soul King and is in the process of absorbing his power, there's very little left for Yhwach to do (unless there something to do with Hell in the future of this arc).
Speaking of the Soul King...
The Horror of God
Back in the Soul King Palace, Yhwach has staggered in the process of absorbing the Soul King. As Lille Barro tries to ask Yhwach if he is alright, these things begin pouring out from Yhwach's body:
In case anyone is wondering, I put together the above image.
As the Mini-Mimihagi begin to swarm out from Yhwach's body, Haschwalth steps in to explain to Lille what they are: a torrent of the Soul King's power that has been at last released from the seals placed upon it by the Shinigami. Because the enormity of the power is such that Soul King/Yhwach (depending on current translations) cannot contain it within their body. As such, they have little choice but to escape in powerful streams.
He also states that they are of no danger to the Stern Ritter, "for the Soul King's enemies are now/only the Shinigami", as the Mini-Mimihagi flow down the black arrow and proceed straight for Seireitei.
Again, it depends on translations and interpretations, but the end result has the same impact. The visuals alone are enough of an indicator for some dark implications regarding the Soul King's true nature.
Looking back on "Deicide", Aizen was speaking nothing but the truth when he called the Soul King "that thing", and I do believe he may also turn out to be right in regards to his rage at Urahara for allowing himself to be controlled by the Soul King. The fact that the Soul King's power alone is flowing out in the form of these small shadow abominations not only demonstrates the vast power possessed by the Soul King, but also its inhuman nature.
Given the appearance of Mimihagi, and the existence of these Mini-Mimihagi, I would not be surprised if the "Human" appearance we have seen the Soul King take on thus far is actually just a shell, and the Soul King's true appearance is as some kind of shadowy abomination of unfathomable power. That would explain the missing limbs and why the Soul King is sealed in a chrysalis - the Chrysalis seals the King's power and contains him within a Human form, and the missing limbs are a further precaution to separate and divide the Soul King's power.
But whoever did all this to the Soul King did so a very long time ago, to such an extent that the Soul King's Right Arm became a folklore figure in the Rukongai, and it is entirely possible that none but the Squad Zero (and Haschwalth) might be aware of the Soul King's true nature.
The fact that Haschwalth knows this much in the first place further demonstrates his unique character and position in the Vandenreich. He knows things that no other Stern Ritter is aware of, and in ways he's more like Yhwach's other half than a separate individual.
But that is enough speculation on my part regarding the Soul King for now...
The Assault of Shadows
Back down in the Seireitei, we see Hisagi asking what is going on, while Soi Fon tells him that it has to be the work of the Quincy, stating that the Shakonmaku has a piece missing, which the Right Arm used to make its way towards the Soul King Palace, and asserts that the gap will become a convenient target for the Quincy.
While she's right about the shroud being the work of the Quincy - Yhwach's work, to be precise - I do believe she's wrong about the gap in the Shakonmaku being a convenient target for the Quincy. Firstly, there are no active Stern Ritter left in the Seireitei (that we know of) who could try to take advantage of it. Secondly, she does not know why the shroud came down (Yhwach wants to keep the Shinigami in the Seireitei).
To punctuate her words, she exits out of the lab and activates her Bankai in order to destroy the shroud. If there's one thing I know about Soi Fon from reading Bleach, is that she has a lot of confidence in herself. She does not know what the shroud is - only that the Quincy are behind it - yet she's confident that she can blow apart that shroud with her Bankai. Given that Yhwach made that shroud with the power of the Soul King's Right Arm, it's going to take a lot more than a giant missile to destroy the shroud.
But she doesn't get the chance to test whether or not she could destroy the shroud, as the Mini-Mimihagi finished their descent and immediately began swarming Soi Fon, who was about to be completely engulfed in the swarm if not for Omaeda's assistance, who she promptly chastise for his concern and hits him for "taking so long". Look Soi Fon, we all know Omaeda's not perfect, but at least show some appreciation for his efforts instead of playing Tsundere to the hilt.
Following immediately behind are Hisagi, Ikkaku and Yumichika. While Ikkaku already has Houzukimaru released, we see Hisagi and Yumichika release their Shikai without needing to utter the release command, which likely indicates that the both of them do indeed have Bankai.
However, they too are quickly swamped by the tide of the Soul King's power, and even Byakuya's Senbonzakura was only able to push back one swarm of them which was only replaced by another.
NOTE: If this is just the normal Senbonzakura, then Byakuya's Shikai received a massive power-up in proportion to Byakuya's own increase in power, since I can only recall him using so many petals before when he was in Bankai.
Given that the Shinigami do not know what these things are, it is likely their attacks are not working because the Mini-Mimihagi are not physical entities, but personifications of the Soul King's vast power. As such, the swarm will continue going for as long as Yhwach continues absorbing the Soul King's power. They might stop when he's completely finished absorbing the Soul King's power, but that's not an immediate solution that the Shinigami can wait around for.
Fortunately for them (sort of), they have help from their worst nightmare...
The Man Who Became God
As it seems like the Shinigami will be overwhelmed, the Mini-Mimihagi are suddenly crushed under an incredible power, as a familiar voice mocks the Shinigami for using their Zanpakuto against the Mini-Mimihagi, when they could simply use their reiatsu to crush them in one fell swoop.
Enter Aizen, sitting on a chair, still in the restraints that keep his reiatsu confined around his body (which doesn't seem to be an issue for Aizen's reiatsu-crushing the Mini-Mimihagi).
You got to hate his smug personalities. He states things like they are something that can obviously be done, but knows there is a distinct difference between himself and his enemies (or allies). It's like he's mocking the Shinigami for needing to bring him out of Muken to help against Yhwach.
We already know that Rukia's shocked at Aizen's return, so that leaves the reactions of everyone else unaccounted for. Doubtless the Visoreds will be very mad to see him again - especially the hot-headed Hiyori. As such, I can only imagine how people will react to learning that Kyoraku is the one who released him from Muken. The Gotei is not going to be happy with its Captain-Commander...