Song to listen to while reading this chapter: "Waking The Demon" by Audiomachine
Well damn, that went better than I expected.
So we start off with Yhwach being a prick and more reaction shots from around the realms, comprising about 3 pages. It's Keigo, Mizuiro, some 'Soldat scrubs, and Bazz-B, who's looking awfully lanky today. Of course, the Humans are still confused as to why there are earthquakes happening, and the Quincies realize they've been betrayed. Classic, really.
Onto the meat of the chapter. Ukitake is indeed using Kamikake (comments' prediction #1 correct) to replace the Soul King (comments' prediction #2 correct). However, in doing so, we get quite a bit of backstory. Turns out, when Ukitake was 3, he contracted a fatal lung disease (which makes it all the more likely that it WAS TB)that turned his hair white. No one could cure it, so his parents took him to a shrine and, well...prayed to what amounts to a pagan god for help. Said god, the titular Mimihagi, who is also the Soul King's right hand, took Ukitake's lungs away, allowing him to live and become a Shinigami. Of course, that resulted in a whole lot of blood coughed up and days on a sickbed, but it's k because divine intervention.
Back in the present, Ukitake is completing the Kamikake, which is actually the process of Mimihagi taking the rest of his body and his life in exchange for filling Mimihagi's right hand position. This results in Ukitake gaining
a really fancy floral tattoo the Kamikake on his chest and the shadow rocketing into the sky as Ukitake screams in pain before it cuts to black. How pleasant.
But wait, we're not done yet, no sirree. Shunsui's in Muken, and as he notes that the tremors are somehow hitting him even here, he calls out to the one you've all been waiting for - AIZEN SOUSUKE.
All in all, a very solid chapter. It seems a lot of people in the comments were right about this being a way to prevent the realms' destruction by replacing the king, and it was nice to get a bit more backstory for Ukitake, as well as a reason behind his "sickness". Also, apparently people can be born from parents in the Rukongai. That's a surprise.