Once upon a time, decades ago, on a planet located across the galaxy… there lived a race of sentient, hostile, purple, mushroom-like aliens, who referred to themselves as the Shroobs. The Shroobs were highly intelligent and innovative, but they were also selfish, relentless, and possessed a sadistic streak a glorpnor wide! Due to this intimidating combination of traits, they out-competed all their other species on their planet and quickly rose to dominate it...for a time. However, their rise to and maintenance of this dominance was not without a high price, as due to their relentless and selfish nature, they came to put more and more strain on the environment of their home world, pushing it closer to the breaking point. Eventually, they realized the gravity of their situation and attempted to revere the damage, but by then it was too late, as it had reached the point of no return, and wholesale environmental collapse was inevitable, collapse which would most likely render the planet wholly uninhabitable, even to the technologically advanced Shroobs themselves.
Desperate to preserve their species, they began scanning the galaxy from afar, searching for a new planet they could travel to and colonize. And soon enough their search paid off, as they found a planet some light years away with an atmosphere which was habitable to them and the appropriate gravitational pull. A planet which, oddly enough, was ALSO inhabited by a race of sentient mushroom people, who they later found out were called Toads. However, unlike the Shroobs, these Toads were a peaceful lot, and had little history of combat in their long rule over the planet. In fact, they were so unaccustomed to fighting that even when urgent matters presented themselves, such as their leader being threatened with imminent abduction, they were unable to provide any meaningful resistance to it due to their lack of combat experience.
This, of course, was perfect for the Shroobs. Not only was this planet the perfect fit for their biology, but that the dominant species of said planet was so inexperienced with war meant that, were they to attempt an invasion, they would be met with, at most, minimal resistance. As such, the decision was made to abandon their home planet and emigrate en masse to this new one, and once they got there, the Shroobs launched an all out assault on the Toads and their kingdom, and quickly brought it to its knees. The nearby 'Koopa Troop', formerly longtime enemy of the Toads, realized the gravity of the situation and tried to counterattack the Shroobs, but alas, too quickly fell, and the Shroobs, satisfied with their victory, began remaking the land and its former capitol castle to their liking in preparation of imminent colonization.
However, just when all hope seemed lost for the besieged kingdom, two heroic brothers named Mario and Luigi came from the future to this time period in an attempt to rescue their Princess, who has ONCE AGAIN gone missing. However, once they got there, they quickly learned about the Shroob invasion afflicting their kingdom's past, and knew they had to do their best to stop it, no matter how slim the odds of their success were. And so, with the help of their younger counterparts, whom they met soon after their arrival in the past, they launched a counterattack against the Shroob invaders, and vowed not to stop fighting until their kingdom was liberated.
The Shroobs, never fools, quickly learned of this resistance force, but also devised several ways to use it to their advantage just as quickly. For one, when the princess the Mario Bros were looking for arrived, she had encountered their elder leader, and trapped her in the power source of the time machine, the Cobalt Star, before scattering the pieces across the land. The Shroobs knew they needed to find and recombine the shards eventually, but since the Bros were collecting them on their own, and seemed to have a knack for knowing where they were, the Shroobs knew it was in their best interest to let them keep at it and only had a few nearby enemies engage them from time to time so the bros wouldn't suspect anything was going on. Meanwhile, the Shroobs put the Princess they were looking for in their home castle and setup a barrier around it which would require the Mario Bros to have most of the shards to enter it. And once they inevitably broke the barrier and entered the castle to rescue their princess, the Shroobs would proceed to ambush them and take the shards away from them. After that, freeing their leader once again was as simple as taking the shards they took containing their Princess to another castle and recombining them with the one they themselves retained. But before that happened, they would have their captured leader's younger sister disguise as their lost princess to hitch a ride back with them to the future for a while. This would allow her to scope it out undetected before returning to the past and reporting what she found, information which would prove useful in their plans to conquer the future Mushroom Kingdom as well.
However, unfortunately for the Shroobs, things did not go according to their plans. For one, the resident villain of the future, Bowser, kidnapped the disguised Princess Shroob and took her back to the past before she could gather anything useful to report to her forces. Secondly, the Shroobs vastly underestimated the skill of the Mario Bros, as when the Younger Princess Shroob confronted them to try and take the shards, they instead defeated her in battle and she quickly died. And making matters worse, when the shards were combined anyways and the Elder Princess Shroob was freed, she too engaged them in battle, but quickly suffered the same defeat and death as her younger sister had. And finally, immediately after the defeat of their leaders, before the Shroobs could even attempt to make a backup plan, the older and younger Mario Bros, through pure chance, discovered the weakness of the Shroob invaders… baby tears, or rather the salt water in them,and saltwater was in no short supply on the surface of this planet. As such, they enlisted the help of the resident ghost-expert and inventor Professor Elvin Gadd, and he quickly devised a machine which would take saltwater and rain it across the land, giving the Shrooh forces nowhere to hide from its toxic effects.
Once this saltwater plan was enacted, the Shroobs, still disorganized from the sudden loss of their two leaders, were unable to combat it effectively despite their superior intellect and technology, and as such, they quickly left this planet, never to be seen there again, (sans for a few individuals Bowser kept cryogenically frozen in his lair for reasons unknown.) However, while the story of the Shroobs ended there for the inhabitants of the Mushroom and other nearby kingdoms, the story of the Shroobs themselves was far from over. After all, the Shroobs, understanding the gravity of the situation before leaving their home planet, had scoped out an additional habitable planet to invade in what they considered the off chance their initial colonization attempt failed. And as such, when their conquest of the Toad's planet failed, they simply retreated before turning their sights to their plan B planet and began making their way there.
This new planet was a world that was healthy and green and could easily sustain the Shroob populace for a long time. It was also a world filled with literal magic, magic which could aid their lives considerably once they found out how to control it, as well as preventing anypony there from trying to repulse them. It was a world run by pastel equines who valued friendship and peace highly, and who as such would likely offer even less resistance to their attack than the Toads had. It was a place simply known to the natives as, Equestria.
This time, there would be fury, and, with the aid of an immunity to the saltwater they developed en route, this dominant species wasn't about to exploit an Achilles heel brought on by a common substance on the planet to bring an end to an otherwise successful invasion, nor would their magic of friendship be successful in repulsing their attack. With their forces poised to attack, the Shroobs sent a few scouts to the ground to scope things out before they began their all out attack...