Fairy Tail has a few of these like Chapter 134 when the Oración Seis takes out the entire guild alliance and Chapter 169, the beginning of the Edolas arc.
The ending of the Edolas arc too. The heroes are back home, having sorted everything out and patting each other back when, "Oh hey, what's the Alternate Universe counterpart of the dead childhood friend doing here?"
And again in Chapter 215, where we learn Master Hades, the leader of Grimoire Heart, is the previous master of Fairy Tail.
To go into further detail: In Chapters 215 & 216: When the series Big Good, Makarov, whom many thought was the strongest character in the series was easilydefeated by the next Big Bad Master Hades. Hades also reveals that Zeref is "the essence of magic."
The latter is repeated again but in reverse when after Hades suffers his own nasty defeat, when low and behold the previously angsty and self-loathing Zeref shows up and, all the while freezing Kain Hikaru and Rustyrose with only his presence, reveals that he has been "awake" all this time, that he values human life as opposed to everyone believing him to be the Big Bad of the series, and that he sees Grimoire Heart and Master Hades as nothing more than trash. He then promptly finishes off Hades for good with one attack, before turning around a feeling something far worse coming over the horizon...
And that something turns out to be the black dragon that kicked Gildarts' ass. The dragon then proceeds to utterly devastate the Fairy Tail mages before destroying the island. Holy shit.
In Chapter 221, apparently Grimoire Heart intends to use Zeref to kill all of the muggles in the world and remake it as a world of wizards... and they've just caught the final ingredient...dammit.
In Chapter 235, Azuma uproots the entire Tenrou Tree, and it is revealed its magic was boosting Fairy Tail's magic, and keeping them alive under the Kin's onslaught. In other words, with it gone, not only are the Fairy Tail wizards being further drained of magic, but it also heavily implies that Makarov and Mirajane really are going to die now.
And Chapter 253... Oh My God, the 253rd Chapter... To elaborate, Acnologia destroys Tenrou Island seemingly along with the entire core cast and many extras, and then has a 7 year time skip. Watch out for the wall!
Note that all of the above happen in the same arc. More like a Wham Arc!
Personal Wham for Lucy in Chapter 256: Her father has been dead for an entire month.
Chapter 297: Glimpses of the near future tell that something quite catastrophic is about to happen. And quite a few people will die from it too...
Chapter 300: Jiemma kills Lector. Sting blasts a hole though Jiemma.
Chapter 301: There was once a Dragon Slayer. That Dragon Slayer killed many Dragons and bathed in their blood. He gradually turned into a Dragon. He is now the Dragon King. That Dragon is Acnologia.
Oh yeah and he possibly had some help from Zeref.
Arcadios and Yukino meet Lucy and the Dragon Slayers.
Chapter 312: The hooded person is from the future...and it's Lucy.
Chapter 314 has a big reveal, as we finally find out why Kagura hates Jellal so much. She is Simon's little sister, wishing to kill Jellal to avenge her brother. And when distraught Erza points put that Simon's death was actually her fault, Kagura slowly pulls out Archenemy...
Chapter 315: Erza saved Kagura from being kidnapped by Zeref's cultists when they were younger.
Chapter 316: Future Lucy wakes up and starts sharing details on what happened. It then crosses over to the Princess who expositions several details, including the apocalyptic event that occurred in the future: The Bad Future will be caused by a herd of 10,000 dragons on their way to attack Fiore and will leave the kingdom in utter ruins.
Chapter 323: Lucy isn't the only person who came from the future, there's also Future!Rogue who was possibly hiding in Present!Rogue's shadow. It Makes Sense in Context.
Chapter 324: We learn that Future!Rogue is from 7 years in the future and that he's on a mission to eliminate the person who will close the Eclipse Gate and lead the country to its destruction. This person is none other than Lucy and as Future!Rogue rushes in for the kill, Future!Lucy makes a Heroic Sacrifice and takes the blow straight on.
Chapter 327: It turns out that due to certain circumstances, the Eclipse Gate, isn't what will save Fiore. Rather, it's what will bring its destruction via 10,000 dragons.
Chapter 333: Present!Rogue finds out that his future self is the one responsible for the destruction of the country and the despair of everyone around him. To make matters worse, Ultear is determined to kill Present!Rogue in order to return everything back to the way it was.
Chapter 334: Gray gets shishkabob-ed by multiple mini-dragon blasts. And it looks like he's going to die....
Chapter 335: Ultear sacrifices her life energy to reverse time back to before the Eclipse Gate opened. It only sets time back by one minute, but this is enough for Gray to dodge the blasts that would have killed him, as well as giving the heroes a chance to turn the tide of the battle to their advantage.
Chapter 337: With the Eclipse Gate destroyed, everything starts unwinding itself, but Mavis is concerned that none of the Dragon Slayers were able to defeat a single dragon. A more potent wham episode is that Future!Rogue informs Natsu that in a year's time, Frosch will be killed.
Chapter 339: Cobra plans to break the Oración Seis out of prison and it's heavily implied the old woman Gray, Jellal, and Merudy encountered was an older Ultear.
The bigger whammy is that it has been confirmed the two know each other. Many fans saw it coming, but to finally see it confirmed is something else.
Chapter 342: The guild encounter the 4th strongest Wizard Saint, who was one of the founding members of Fairy Tail.
Chapter 343: Minerva is back and this time, she is a Dark Guild member.
Chapter 344: Erza's been turned into a child.
Chapter 345: And now Natsu has been reverted to a child, and the one responsible is a member of Succubus Eye.
Chapter 348: Gray has been reverted to a child, and the mystery man mentions two key things: "The other side of the gate of Hades" and taking credit for freezing the village.
Chapter 349: Turns out the mystery man, Doriate, was lying about freezing the village, but then he reveals himself to be a demon from the Book of Zeref like Deliora and Lullaby.
Chapter 352: The Eternal Flame has been revived and his identity has been revealed. It's Atlas Flame.
To stress how whammy this is, NATSU MEETS A DRAGON FROM THE PRESENT!
Chapter 352 also cruelly subverts this: It is only a memory summoned by Wendy.
Chapter 356: Tartarus make their move, the Magic Council is wiped out and it is revealed that Tartarus is made up of demons from the Book of Zeref, led by E.N.D., the strongest of Zeref's demons, whom Igneel failed to defeat.
Chapter 358. Just when you thought this arc couldn't get any Darker and Edgier, Tempesta's Taking You with Me move creates the magical equivalent of a chemical weapons attack, killing hundreds of Hargeon's citizens and nearly kills the Raijin Tribe. It ends with Natsu promising a Roaring Rampage of Revenge.
Chapter 360: Jackal's power manages to defeat Natsu, who's been built up as nearly invincible at this point.
Chapter 363: We learn why Tartarus is slaughtering the Magic Council; the Council possesses a weapon called Face capable of nullifying all magic across the continent. This will leave all mages powerless, while Tartarus, using curses instead of magic, will be free to walk all over them. Three unknown former Council members are the living keys to the seal of Face, so Tartarus is indiscriminately killing every Council member until the three keys are dead.
Chapter 364, full stop. For one thing, the Oración Seis is back; turns out Doranbolt had to free them in exchange for Cobra's intel on Tartarus. He also hired Jellal and Meredy to keep an eye on them, knowing full well how risky of a move that is. And then we have Crawford Seam, the ex-chairman of the Magic Council and an all-around Fat Bastard who only very rarely made appearances in the series, return... as The Mole for Tartarus, who is now kidnapping Erza and Mirajane and delivering their unconscious bodies to Tartarus' HQ. And last but not least, Elfman may now be forced to sell his soul in a Deal with the Devil in order for Seliah to spare Lisanna's life. Darker and Edgier: The Arc indeed.
Chapter 365: As Erza is being tortured by Kyouka at Tartarus' HQ, we find out that two of the three keys to the seal of Face are already dead. The third key... is Jellal.
Chapter 368: We find out the details of Elfman's Deal with the Devil: He is being forced to install a lacrima bomb 500 times more powerful than Jupiter Cannon that will instantly vaporize Fairy Tail. What makes this worse is that he's fully aware of what he is doing, but because of Seliah's Macro he is unable to do a thing about it.
Jellal gets taken out by Zero.
Chapter 369: Zero ends up being an illusion created by Midnight. To counter this, Jellal blinds himself on purpose and takes out all of the Oración Seis with his Sema spell. After their defeat, he invites the members of the Oración Seis to join Crime Sorcičre and defeat Zeref. At the same time, Crawford Seam, the former Chairman of the Magic Council successfully transfers Jellal's key to himself and before he can transfer the key off to someone else, he is unceremoniouslykilled off by KyoukaIn the Back. With his death, the seal on Face has been released. The chapter ends with a teary eyed Elfman setting off the lacrima bomb in the Fairy Tail guild hall.