Warning: To avoid a completely spoiler-tagged blank page, all spoilers have been left unmarked, be warned and tread carefully
The biggest one, by far, is that Aizen not only faked his death and took part in the conspiracy, he's the mastermind behind it all. The entirety of Bleach from here on would never be the same.
The Soul Society arc sets the Soul Society's laws up as the problem, given a face by Byakuya, whom was dead-set on killing his sister, Rukia. And then he takes the bullet for her when Gin tries to kill her. When asked why, he reveals that his wife Hisana (who looks like Rukia), was actually Rukia's sister, and he adopted her into the family to fulfill a promise he made to a dying Hisana. The reason he was set on executing her was not because of the law, but because he also made a promise to his dead parents to uphold the law.
After debuting as a cat with a masculine voice, it was rather surprising to see Yoruichi Shihoin turn out to be a female human, and a former captain of the Gotei 13, at that.
Ganju spoke a lot (and spoke well) of his brother Kaien, so when it was revealed that Rukia killed him, it was quite a shock. In fact, her guilt over the Mercy Kill of Kaien was a large part of why she went along with Byakuya and Renji to begin with.
Isshin Kurosaki was once thought of as little more than an eccentric pervert who happened to be Ichigo's father. A couple years later, we see him carrying a Zapakuto while wearing a captains' robe, showing off his power level by easily disposing of former Arc Villain Grand Fisher. That, and he's acquainted with Kisuke Urahara, Yoruichi Shihoin and Ryuken Ishida.
Speaking of Ryuken, one of the first things we learn about him is how he refused to be a Quincy "because there's no profit." Years later, he actually saves his son Uryu from an Arrancar, and takes it upon himself to restore Uryu's Quincy powers.
Ikkaku Madarame, a mere 3rd Seat, was wielding Bankai. Talk about Almighty Janitor.
Introduced as a Tagalong Kid, Nnoitra reveals that Nelliel Tu Odelschwanck was once an Espada. After she forces herself back into her adult form, we see that she was once Espada number three, and she's still a better fighter than Nnoitra.
Ulquiorra was once built up as the strongest Espada under Aizen's command, to the point where Ichigo even asks if he's the Primera. Ulquiorra calmly reveals that he's only the fourth strongest.
Chapter 322. Plucky Comic Relief Yumichika Ayasegawa.... isn't. In fact, he's so powerful he can one-shot fracciones that are making lieutenants struggle. And the only reason he's not using his power has nothing to do with narcissism. It's because of Undying Loyalty to his division's code of conduct. Ouch.
Chapter 392. Hitsugaya stabs Aizen, only for Ichigo to reveal that it was illusion. Instead, he learns that he stabbed Hinamori, thus allowing Aizen to single-handedly take out Hirako, Hitsugaya, Soifon and Kyoraku, all at once.
Aizen moving his shirt away, showing that he fused the Hogyoku within himself, granting him immortality and the ability to go One-Winged Angelseveral times.
A rare subversion comes along here. Way back, we learn that Kaien Shiba was once Rukia's lieutenant, and she had to kill him in order to save him from a Hollow. That's not the Wham Episode. That comes when Aaroniero Arruruerie unmasked himself, revealing Kaien's face. It takes a while until we learn that Aaroniero was only posing as Kaien, quite literally using his corpse as a way to catch Rukia off-guard.
Isshin suddenly showing up in his Soul Reaper form, right in front of Ichigo. He not only decides to fight Aizen, he flung Aizen straight through a building, proving that yes, he really is captain-level.
Chapter 415. Gin's betrayal of Aizen reveals over a century's worth of vengeance plotting, because pieces of Rangiku's soul was used to fuel the Hogyoku.
We learn that there were two Hogyokus, both separately created by Aizen and Urahara. Aizen set up the entire conspiracy in the Soul Society arc to steal Urahara's Hogyoku and fuse it with his own, as neither was complete or even capable of being complete on its own.
The reveal that Tsukishima bookmarked Karin, Yuzu, Ikumi, Tatsuki, Keigo and Mizuiro, effectively brainwashing half of Ichigo's loved ones and forced him to seek help back at Xcution. Until it's revealed that Xcution, Orihime and Sado were all bookmarked as well, leaving Ichigo only with Ginjo for help.
Chapter 458. The Arc Villain is revealed to be Ginjou, not Tsukishima.
Chapter 460: The full extent of how Ichigo has changed the Soul Society is revealed here. The order to break the law and directly interfere with a human's future by giving him shinigami power... comes from Yamamoto himself.
Thousand Year Blood War Arc
The Thousand Year Blood War has been filled with so many different wham episodes that it might as well be said the entire arc is a wham episode in its own right.
Chapter 480: The biggest reveal since Aizen's betrayal is that the Quincies are not dead.
The rather brutal murder of Chojiro Sasakibe, proving that yes, good guys can and will die, even in Bleach.
Thousands of Rukongai dwellers have been disappearing for an unknown reason. Kira deduces, and Mayuri confirms, that Shinigami killed those civilians, in order to restore the balance between the worlds. It hammers home just how morally ambiguous the Soul Society can get, and it returns Mayuri back to form instead of having him as comic relief.
Chapter 485: Hueco Mundo is no longer under the control of the Arrancar. It's fallen to Vandenreich occupation. Furthermore, we see that Harribel has been captured by the Vandenreich, effectively serving as a warning to all who venture near Yhwach's turf.
Yhwach decides to attack the Soul Society on the day of his declaration of war, without even bothering to wait the five days he initially specified.
Chapter 499: For the first time in the entire story, the Soul Society openly seeks help from Ichigo, after we learn that a third of the Gotei 13's forces have been decimated by thousands in the space of only a few minutes. Not even Aizen created this much havoc.
Chapter 502: Byakuya, previously thought by the fandom to have unassailable plot armour, is curb-stomped in one of the most graphic attacks in the entire manga, followed by Kenpachi in Chapter 505.
Chapter 511: Yamamoto is slaughtered and the Seireitei is overrun by the Stern Ritter's ground troops, officially defeated.
Chapter 514: Ichigo's time in Opie's prison actually awakened his innate, but largely dormant, Quincy energy, which is why he could use Blut Vene.
Chapter 516: Broken bankai can never be restored to their original form, thereby redefining the power levels of every shinigami that has experienced a broken bankai in the story, including Mayuri, Ikkaku, Renji, and Ichigo. Komamura's the only exception to the rule because of the way his bankai works. Even though the damage done to it is reflected on him, if he undergoes healing it will fully restore his bankai.
Chapter 518: After spending years missing in action from the story, Kuukaku and Ganju not only reappear, they're the ones that send Squad Zero into the Royal Realm. Not too long after that, we see Ginjo, Tsukishima and Kutsuzawa have passed on to Soul Society, and are now training with Kukaku.
Chapter 520: Shunsui Kyouraku, once the Lazy Bum, takes over as the new captain-commander, forcing Central 46 to allow for Zaraki to fully train with Unohana. Furthermore, he reveals that she didn't always go by the name "Retsu". At one time, she renamed herself "Yachiru".
Chapter 521: Hikifune was the creator of the Gikon and Modified Souls like Kon, and Nimaiya is the creator of the Zanpakuto.
Chapter 523: Kyoraku confirms that Unohana was the first Kenpachi, founding the 11th Division and laying the work for everything it's built on.
Chapter 527: By defeating Unohana, Zaraki learns his zanpakutou's name. Meanwhile, Ichigo fails his training this time around, meaning that his zanpakutou will not be fixed. He's kicked out of the Soul Society until further notice.
Everything But the Rain
Just like the main arc of the Blood War, "Everything But the Rain" is a wham episode in its entirety. The amount of stuff we learn is more than what most other manga can accomplish in the same period of time.
Chapter 528: Remember when Isshin lamented how Ryuken doesn't often call him "Kurosaki?" We learn that Masaki was already known as "Kurosaki" by the time she met Isshin; he adopted her family name over his own. Now, the big revelation? Masaki was a Quincy. It was already guessed by the fans at this point, but we at least have official confirmation.
Chapter 529: Isshin was the captain of the Tenth Division and his surname was Shiba. Two popular fan theories turned out to be true. Plus, Rangiku was his lieutenant (even back then) and Hitsugaya was the 3rd Seat.
Chapter 530: Combined with a Wham Shot. At the very end, we see a creature bearing Ichigo's Hollow mask.
Chapter 531: That creature from earlier? It's the progenitor to the Visoreds and Arrancars.
Chapter 532: Masaki kills White, on her own, without help, after letting it bite her. It establishes that Masaki is not to be trifled with, and it reveals why Ichigo was able to use Fullbring (e.g. mother needs to come in contact with Hollow).
Chapter 534: Masaki starts to develop a Hollow hole as a result of the proto-Arrancar's "target Hollowfication" powers.
Chapter 535: Perhaps the biggest info-dump of this mini-arc thus far. We finally learn how Isshin gave up his Shinigami powers (he wears the same power-sealing type of Gigai that Urahara gave to Rukia). We learn more about Hollowfication (it needs an outside force to disrupt it so you can restore sanity). We learn why it took so long before the Visoreds could show up in the main storyline (it took the full 100 years before Urahara restored their sanity, leaving them with a solid 20 years to gain notoriety and stabilize themselves via training). We learn why the Hollowfication was subdued for so long (the Gigai literally ties his soul to Masaki, which allows him to enter her mind and subdue the Hollow). We learn how the "soul suicide" resulting from Hollowfication can be stopped (which created the Visoreds). In an overlap of the soul suicide revelation, we also find out why Tosen blew up after he was stabbed by Hisagi (didn't have something to counterbalance his Visored form).
Chapter 537 is a double-whammy. Katagiri (confirmed to be Uryu's mother) and Masaki were both killed by Yhwach, who enacted The Purge on all "impure" Quincies so he could take their powers. While Grand Fisher was still the culprit in Masaki's murder, she would've mopped the floor with him had it not been for Yhwach stealing Blut from her. The second one? Uryu Ishida has joined the Vandenreich.
The Thousand Year Blood War: The Farewell Story
Chapter 539: For Ichigo to get his Zanpakuto fixed, he has to abandon Zangetsu. Meanwhile, Komamura's revealed to be part of a unique secret race that's in hiding, that possesses a great secret that can allow Komamura to compensate for his loss of Bankai, and which carries the knowledge of the truth about Soul Society: that Shinigami are not the original rulers of Soul Society.
Chapter 540: Zangetsu isn't actually Ichigo's Zanpakuto. He's the personification of Ichigo's Quincy powers, which have taken the form of a younger Yhwach from 1000 years ago. Furthermore, it's the Hollow that's Ichigo's real Zanpakuto.
Chapter 541: Fake Zangetsu admits he's been suppressing Ichigo's real power all along, because he didn't want him to become a Shinigami. AND he even wanted to kill Ichigo with his own hands, should that ever happen. Good thing he changed his mind.
Chapter 543: The entire chapter changes the view and flavour of the Vandenreich completely. Remember when Opie mentioned Soken and Uryuu Ishida by name? Turns out, Opie was the only one to know of either. We learn of this through Yhwach nominating Uryu as his successor (over Haschwalth, the previous favorite). The army is so taken aback by Yhwach's choice as to suggest that the troops are loyal not to Yhwach, but to Haschwalth.
Chapter 544: Yhwach reveals his reasoning for what happened in the last chapter. Uryu was the only mixed-blood to survive the Holy Selection, because of a mysterious special power that Yhwach wants to know more about. Haschwalth deduces a second reason: Yhwach is expecting Uryuu to betray him, and Yhwach believes in keeping your friends close and your enemies even closer. Even if Ishida was planning to mount an assassination, the scrutiny from the Stern Ritters will prevent him from doing so.
Chapter 546: The Seireitei vanishes into shadow to be replaced by the vandenreich's city and Yhwach reveals that the old Quincy tale of the Sealed King has a stanza that isn't commonly recited: He reclaims his heartbeat, mind and his power, alright... and then he reclaims his world. Yes, apparently the Soul Society did once belong to Yhwach.
Chapter 547: The Seireitei flat-out disappears. Meaning, it effectively no longer exists; it's been turned into a bunch of shadows and it was replaced, fully, by Yhwach's city. The explanation? Yhwach describes how there was a space between the living world and the Soul Society; this space was littered with Reishi. After the war, the Quincies fled to that space and manipulated its Reishi to create a city within the shadows underneath the Seireitei; they've been there the entire time the series has gone on. The reason they bypassed the giant shield in the first assault was because they were already within the shield to begin with.
Chapter 554: Turns out that by stealing back their Bankai, the Shinigami have returned the Stern Ritters their ability to unleash their Vollstandig once more. And seven of them proceed to do just that. Oh, and Yhwach planned for all this to happen.
Chapter 556: Komamura's past is revealed! He comes from a family of humanoids who were turned into animals as punishment for some unknown sins committed in the past. In exchange for beating his ancestor, he learns about the secret family technique - rip out your heart and you will cross the Bishonen Line and become human.
Chapter 558: The chapter contains a very tragic one. The Human Tranformation Technique wears off, and we find out the compensation. Komamura becomes an ordinary wolf, not an anthro. His entire clan is made for revenge, the very thing Sajin warned Tousen against, and ironcially the thing he fell victim to in response to Yamamoto's death. This effectivly kills off his character and more or less means his days of living like a human AND his Shinigami career are over.
Chapter 565: Yhwach's backstory and powers are revealed. He has an ability to distribute pieces of his soul into the souls of other individuals, and when they die, those pieces return to Yhwach with all the memories, experience and powers that those people possessed in life. By doing so, he can make himself more powerful and prolong his own life. This is how Quincies were created, but most importantly, the whole Gotei has now been infected with pieces of Yhwach's soul... meaning EVERY death on the battlefield increases Yhwach's power.