This was when I was into Ninja Turtles. So if you know them hopefully this makes sense.

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Arya lived on a peaceful planet with her family. When she was young the Kraang invaded and destroyed her world. After a great battle she was one of the few survivors and they set out to find a new home. They found earth put realized it was already occupied by the humans. After a long search, they made a city in the Bermuda Triangle and created defenses to keep people away.

Unfortunately that just created the myth and story of Atlantis. Arya became an engineer and left to find more survivors. Her and the others flew their spaceships around to find them. This created the UFO sightings. They realized they were causing a problem with the people of earth and simply remained hidden.

Years later she discovered the Kraang were now on the planet as well. Her and the others wanted revenge and rid this world of them. But they were outnumbered. The leader found a new home and they decided the best thing to do, was destroy the planet altogether. It would free them from their enemy and save the universe.

Destroying one planet would be all they needed to do. They then got to work and built a weapon that would drill to the core. Once there they could expand it and blow up the planet.

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Aaaaand that's about it. :)