Disclaimer: this is just an early draft, so I’ll likely change some things before I submit this. I am also taking constructive criticism in order to improve this in any way I can.
Long ago, far before most anyone could remember, a group of creative spirits known as the Colossals decided to build a great world upon the base that the Big Clangor had created. Sollum molded the land, Grennicth grew the plants, Zeffree filled the sky with air, Hyddryd expended the seas, and Frigil cooled the temperature to a comfortable level. The brainiest of them all, Zarroë, decided that their world should not be home to the Colossals alone, so they volunteered to allow the others to use their form to experiment with creating life within the Pocket Dimension. Their first few experiments, headed by the Colossal Astroogh, proved to be unsuccessful. After future, more successful creations, headed by someone else, resulted in more stable creations, they spread throughout the monster world to inhabit it.
Around this time, the Colossal Astroogh made contact with a group of monsters who dated back to before the Colossals had even arrived. This group, who called themselves the Celestials, quickly developed a friendship with Astroogh, who excitedly introduced their new friends to the other Colossals. It was decided that each Celestial would be assigned a Colossal mentor to teach them how to sing and play music. Rather than being assigned a Celestial themself, however, Astroogh was tasked with building the Starhenge on which the Celestials would live. This task was difficult and tedious but Astroogh pulled through and the newly musical Celestials were placed upon the Starhenge high in the sky.
Seeing that all their celestial companions had become closer friends with other Colossals, and the lack of kin of their own, the lonely Colossal Astroogh began to shout out in frustration. This shout echoed throughout the monster world for a long, long time, unheard by the monsters who continued to sing their wonderful song in contrast to Astroogh’s frustrated shouts. The seemingly sentient Prism Gate, who could hear the Astroogh’s shouting, began to grow irritated by the near constant grating noise. After trying and trying to ignore it for so long, something snapped within whatever controlled the Prism gate, causing it to open its gaping maw across the monster world in an attempt to absorb all within it to finally silence the shouting. This shattered the Continent and absorbed the Outer islands, forcing the majority of the Colossals to inhabit the broken land masses in order to save their creations.
When Astroogh finally stopped shouting, it looked on in terror to the fragmented world and realized what it had done. They quickly made the realization that with the continent shattered, the Starhenge would eventually run out of power and fall back down to the ground. The Astroogh began following closely under the Starhenge, hoping to perhaps catch it when it fell to hopefully save its friends and maybe, just maybe, redeem themselves for what they had done.
Also, for those who care, this is canon to my MSM AU no matter if it wins or not.