So, I've been toying with some ideas on a campaign I run.

I want to hit the group (6 lvl 12-13s) with a cataclysmic devestation to the land they live on. Vastly changing the world they are exploring.

What are some ideas for how This event can happen? What monster or inspirations should I look at.

something that can take out a large portion of the country style devestation. That way while we can wrap up the next 7 levels. I would like the story to direct them into becoming the ground work of the next gen... I'm thinking an advancement of 100 years... 

Honestly I've hit a point in which I want to restructure my world but not start from scratch. So any help would be great...

You can have a meteor strike when the PCs are in another dimension, there can be a powerful Lich who takes residence close and all life within 100 miles dies. You could have a "rare planetary intersection" in which their world and another realm have portals spawn over the place, making the land look like that plane. Dormant volcanoes activate, a Curse (like the one in Tomb of Annihilation) spreads across the land, or you could do what I'm planning on doing in my campaign: Uprooted. There are so many options. Honestly, your imagination is the limit. I hope you like my suggestions!

 Thanks for the ideas. I'm literally drawing blanks 

There are a lot of different ways to do something like this, depending on how much involvement you want your characters to have in it and the magic level of your campaign.

Things like:

• A high level wizard made a mistake and caused a large magical explosion,  this has whatever effects on the land within whatever radius of where the mistake happened.
• Some other group (not your party) in the game world failed to stop something from happening, Ie the return of an ancient dragon, or a demon/devil other horrible creature
• A natural disaster (flood/meteor/volcano) perhaps induced by other events that happened in your game world.

Those are just a few quick thoughts, hope that helps.

I actually combined 2 of the previous ideas into one and made it the plot hook for one of my current games: 

The mage's academy is celebrating the next generation of students graduating from the academy. During the celebration a major explosion rips through the town, destroying everything in a large area. It becomes the equivalent of a magical nuclear wasteland. The strange part is, during the ensuing chaos, one of the players noticed a strange humanoid figure that, when it touched another person, caused the person they touched to die almost instantly. Basically the group witnessed a mistake; magic gone wrong creating the magical wasteland. The result was a mage, unknowingly, being turned into a Lich by accident as well.

A creeping darkness is spreading over the land. At first people believe that a warlock has used her vile powers to enhance or activate a pair of volcanoes, causing them to spew thick clouds of ash into the sky.

If the players succeed in stopping her, they discover that the darkness still remains. The combination of volcanic ash and the magical darkness are causing an unnatural winter, even if it's (whatever season). What is causing it? Where will it stop? How will the people survive if they cannot grow anything?

This recently occurred in one of the games I'm running.

The original inspiration for the campaign was to take the Phandalin adventure from the introductory set to 5e and then merge in Out of the Abyss by having the players get dropped into the Underdark after fighting off the evil wizard, with a bunch of home-brewed stuff here and ripping out entire chunks of things and putting in my own concept of them there. The important thing about the backstory is that Out of the Abyss heavily features Demon Lords, and I switched them out with Dead Gods who were prophesied to return to life shortly, bringing about the equivalent of Ragnarok/Armageddon/etc.

Well, my players being who they are decided to screw that entire plan into the ground by founding a town of refugees after they accidentally burned down Phandalin and started a war, so I decided against sending them down into the Underdark and instead allowed them to pursue town-building for a while. This explains why there were still on the surface when the Conjunction (the bringing back of the Dead Gods [and the re-conjoining of the Abyss/Underdark with the plane they live on]) began to occur.

One of the players is playing a Wild Mage Sorcerer who was trying to find out why his (and his father's) magic was all screwed up. During their latest adventure, during which he found out his family had been subjected to horrible experiments by a mad doctor trying to exploit Wild Magic in order to discover new spells, they discovered that he's actually related to the mortal form of someone who may or may not be a Dead God, which partially explains why the Wild Magic stuff is happening to him. Anyways, that's all to say that magic behaves really weirdly for him and sometimes does completely unexpected things - and the location they were in was heightening some of that weirdness due to the mad doctor's experiments.

The party encountered some trees that animated and started attacking them while they were trudging through a Spooky Forest. This sorcerer and a Cleric of one of those Dead Gods then proceeded to accidentally set the entire forest on fire through the use of Flame Bolt, Fireball, and Call Lightning spells. Some wacky magical hijinks ensued and the storms (both the firestorm and the lightning storm and the fact that it was already stormy conditions anyways) attracted the attention of one of the Dead Gods, who unbeknownst to the world at large are no longer Dead for reasons as yet unrevealed, and, well, things got out of hand.

The wildfire destroyed much of the (not small) island the party were on at the time (and half the party nearly died escaping/surviving it, the other half having a much nicer time in a completely different location after the storm cleric accidentally split the party), and a number of other odd magical effects occurred. One thing to know is that the Dead God that was called over isn't just the God of Storms; he's also the God of Time, and a time-storm occurred in the same location. The storms grew until they were hurricane proportion, magic went haywire, time was awful, and so on.

Long story short - the entire landscape, both political and geological, was changed due to the confluence of Wild Magic and Happenings of Religious Significance.

There are also earthquakes, the merging of another plane into the one they live on, and a number of other things that are happening that the players aren't entirely clued in on yet that're going to drastically change the world.

Could be the classics. meteor, volcano, tsunami. Could be magic gone wrong. Could be the Gods. It really depends where you want to go with it.

Maybe.... Eadro has been working with the Locathah, Merfolk and Sahuagin to undermine the continent, plunging it into the ocean and establishing a new underwater kingdom, because underwater volcanic activity (which is the cause of the tsunami's) has destroyed previously safe havens, changing current and fish feeding grounds.

In the campaign I'm writing, I plan on a lich using a huge monstrosity to destroy the continent they live on as the end. If the lich were to succeeds, then (in theory) huge amounts of people die that he can use to make a deal with Orcus to basically make the Material Plane a vassal of the Abyss (its D&D, it doesn't need to make 100% sense). The liches calculations were off and the world is still destroyed, just not infested with Demons and Undead. 

It were to only happen if weak players (lvl 10-15) fail to stop a CR 21 creature from summoning a weird Eldritch horror. If they survive the ensuring chaos, then it's their job to find survivors and rebuild their once proud contenant, now a bunch of scattered boulders/islands in the ocean.

A plague is a great way to add some spice to your campaign, or just to wipe out half the population. Consider making the disease resistant to healing magic if clerics and healers are common in your world. There is a great article on here I would suggest reading. 

The best world shattering event I know of in gaming fantasy is clearly the cataclysm of Krynn. The gods loose patience with a decadent mortal empire, slam a comet into the world and all clerical magic stops working; good dragons vanish. People loose faith in the gods and the world map is redrawn. Then the evil dragon goddess Takhisis tries to take over the world, setting the stage for all sorts of possible adventures and quests. The time line is advanced too you could say. It should be something along those lines -- something that the players can get easily, is believable and makes sense (from a D&D viewpoint). 

In my opinion, the best canonical DnD Cataclysm was the Spellplauge. You could have your characters watching the devastation from a protected wizards tower. Although, this might be best for non-magical parties, as the Spellplauge stopped magic for like a decade.

a time traveler from the 1980's comes and BLodjns up some magical device and , like causes a magci network error and magci starts malfunctioning!

I had an iron meteorite devastate a large area. That allowed me to:
1. have a side quest, where the party gained the favour of a community by conducting (what we would think of as) search-and-rescue operations in the aftermath; and 
2. they found the remains of the meteorite, and had to negotiate with orcs over who got to keep the valuable metal inside it.

As a side benefit, it also gave me a reason to make a miniature of an iron meteorite out of air-dry clay, which I was quite pleased with.

4 tarrasques are released by the villain whist they are in a dungeon/ alternate dimension option 2 is the big bad finds an ultra powerful artifact and temporary looses control of their powers, drastically changing the land.

You could have a demon lord conquering the land and scorching it with infernal flames with army’s of the demon lord that is invading roaming around the land 

I mean, in a living, breathing campaign world the world is currently being altered. In Lost Mine of Phandelver, and Dragon of Icespire peak there are enough players to take some small imagination and kick off an election for a new townmaster. This could devolve down into a civil war should you wish.

Likewise, you could go down the natural disaster route. Maybe the party experience an earthquake. Something small like a tremour. Throw them in every now and again. Once you've dropped enough have an actual large scale earthquake damage lots of buildings in and around a nearby town. Follow this up with a massive volcano eruption. I'd be considering a Java level eruption here. black skies, pyuroclastic clouds that choke wildlife and make the air difficult to breath. The thick black clouds block out the sun allowing creatures from the underdark to begin emerging and preying on the fleeing population.

You could go down the route of time travel. Portals don't have to take you from one place to another, but could shift you in time. This might find your players in a world that has an entirely new world order (think the children returning to Narnia in Prince Caspian...their old castles ruined, towns destroyed etc.)

Do you have a warlock who doesn't yet know what the terms of their pact are? You could let the player be given a task by their patron - kill an innocent. When they refuse, the patron is brought to this plane and wreaks havoc upon the world. 

You could put a fiend or celestial in the way of the party - once defeated that creatures 'superiors' could do an 'Angel' and send the entire region to literal hell.

Finally there's the shipping the party off tactic. Throw the party on a ship and send them somewhere else. By the time they come back there's a mystery to solve - why has the world they know changed so much?

Your (maybe next) BBEG actually succeeds in their plan... 

What I would do for a world-altering event is something like this as a being tries to become a lich and fails Things go terribly wrong a Massive Rip in space opens letting out a corruption that is taking over creatures in the nearby vicinity turning them Twisted and wrong orcs having blackish-purple flesh spewing out of parts in there body making them loose any control and becoming basically zombies