The Marvel Cinematic Universe sure does feature a whole load of catastrophes large and not-as-large.
Successful
- Class 6, not that far off from a Class X: in Guardians of the Galaxy, the Collector shows recordings of the Power Stone being used by a Celestial on at least one sapient world in the past, unleashing a World-Wrecking Wave which turns a relatively-green world into a cracked cinder scorched with the stone's signature energy.
- Thanos's home planet Titan suffered one somewhere between Class 4 and Class 6, which reduced it from an idyllic world to a stormy-skied, ruin-littered wasteland, as revealed in Avengers: Infinity War. Thanos claims that an Overpopulation Crisis caused the extinction event, but Peter (Quill) notes that the planet itself is off its axis and that gravitational pull is "all over the place", indicating that a lack of resources wasn't the only reason for Titan's demise.
- Class X: Calling Asgard a planet in its own right might be a stretch, but in Thor: Ragnarok, it's ravaged by Surtr's fire and finally blasted to pieces, completing the prophecy of Ragnarök. Fortunately, a handful of Asgardians escape and survive. The Sacred Timeline isn't the only universe where Asgard meets this kind of end either.
- Class 1: Thanos inflicts this on every inhabited planet in the universe with the Snap in Avengers: Infinity War, causing half of all sapient and animal life to near-spontaneously turn to dust at random. Avengers: Endgame shows the aftermath on Earth, with cities and suburban neighborhoods around the world looking near-deserted, world governments "in pieces", and the surviving half of the population deeply traumatized — one gets the implication that it could have slid into total collapse without the surviving Avengers doing all that they can to help the world hold together. Most of the planets in the universe are implied to have it at least as bad as Earth if not worse, with Captain Marvel trying to pick up the slack on her own.
- Class X-4: In "What If... Doctor Strange Lost His Heart Instead of His Hands?", Strange Supreme triggers a Reality-Breaking Paradox which ultimately destroys his entire universe and everyone in it but himself, causing it to collapse around him into a tiny crystallized remnant.
- Infinity Ultron unleashes a bunch of them all over his native universe in "What If... Ultron Won?". First, he inflicts at least a Class 4 on Earth via total nuclear holocaust (probably a Class 5, based on Clint and Natasha's lack of even animal companionship and lack of views of what the countryside looks like). Then after Ultron takes his genocidal campaign universal, he inflicts a series of Class X's on multiple familiar worlds — and at one point he even creates a blast of power that seemingly causes a Class X-2 to Xandar's star system — until all sapient life (possibly even all life of any kind period) in his universe is functionally extinct, with only Hawkeye and Black Widow being known to survive.
- Class X-3: And after Infinity Ultron escapes into the multiverse, in one universe he seemingly causes this when he chomps down on an entire galaxy. There's no telling just how much devastation to how many universes he managed to cause before the Guardians of the Multiverse stopped him for good.
- The backstory of Loki (2021) reveals that the current iteration of reality, the "Sacred Timeline", is set after a Class X-5. Long ago, there once raged a Great Multiversal War, presumably involving all variations of Kang the Conqueror, and the last survivor (a Kang variant going by the name "He Who Remains") created the Time Variance Authority to prune any and all derivations from the Sacred Timeline so that another, possibly crueler Kang, isn't created and result in another Multiversal War (which, according to He Who Remains himself, would have as its end result "everything burning"). This gets undone when Sylvie kills He Who Remains, causing the Sacred Timeline to erupt into numerous different timelines and re-creating the Multiverse.
Attempted
- In Thor, Loki attempts to turn the Bifrost Bridge on Jötunheim to cause a Class X to the planet and eradicate the Frost Giants.
- During Thor: The Dark World, Malekith the Accursed and the Dark Elves tried twice to use the Aether to plunge all the cosmos into an everlasting night, bringing death and decay across the universe. This would be a Class X-4.
- In Guardians of the Galaxy, Ronan intends to use the Power Stone to inflict a Class 6 on Xandar and all its inhabitants, just as the Stone was used long ago to inflict such an apocalypse on other worlds. And he comes scarily close to success.
- Class 4: In Avengers: Age of Ultron, Ultron intends to inflict a K-T level extinction event by turning Sokovia into a Chicxulub-like impactor for a Colony Drop. He at first indicates he wants to push humanity to evolve and improve (in which case, it would likely be a Class 2 for the human race alone), but his reasoning is Insane Troll Logic at best with his fractured mindset. Friday states when Ultron is enacting his plan that the blast will likely cause "global extinction", and after losing Vision, Ultron implies he'll probably try to inflict a Class 5 so that he's the only thing left alive.
- Ego the Living Planet's Assimilation Plot to remake every inhabited world into an extension of himself, as revealed in Guardians of the Galaxy Vol. 2, would likely spell a series of universe-wide Class 6s. The seeds of himself that he's planted on the "thousands of worlds" he's visited (which include Earth) are shown aggressively turning into very fast-spreading Blob Monsters which crush everything in their path, and they're specifically intended by their creator to cover every inch of those planets' surfaces.
- In Thor: Ragnarok, Hela getting what she wants would spell very bad news (read: multiple Class 2s) for any of the Nine Realms, given how much she's looking forward to "drenching them in blood" in the name of conquest.
- Class X-4: In Avengers: Endgame after seeing that his plan ultimately fails, a past version of Thanos comes to the present with a new plan; to use the Infinity Stones to "shred this universe down to its last atom" and make a new one the way he envisioned, with nobody from the old one besides him alive to remember the way things used to be.
- Once Infinity Ultron escapes from his own universe into the multiverse, he intends to repeat his systematic genocide of all sapient life upon all the other universes, resulting in a Class X-5.
- At the climax of Moon Knight, it's revealed that Arthur Harrow's Evil Plan is to reawaken his goddess Ammit so she will extend his Vampiric Draining Precrime Arrest powers to all of his fellow cultists so they can all collectively "judge humanity and make a Heaven on Earth". Considering how the Cult of Ammit seems to consists of at least several thousand people scattered all across the world and Ammit herself makes it clear that she desires to "judge" the whole planet, it's all but outright stated that Ammit and Harrow's plan — if not thwarted — could cause a Class 2 across the Earth.
