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More horse!!!! Been trying to get myself to design her armor for a while and i think i like what i ended up with. Anyway, headcanon time! sorry this is long haha
 
Sunset Shimmer in this HC is Sunbursts younger sister by a few years, and knew Starlight Glimmer a little through her brother. After Sunburst left to study abroad, Sunset became standoffish as a young child and would often try to start fights. It wasnt much later that she encountered a powerful unicorn noble visiting from manehattan, and being a little shit, she started negging him and trying to ruin his visit to the (at least to him) unimportant town. Seeing an opportunity to make an example, the noble challenged her to a magic duel, with the Sun as witness, envoking a traditional unicorn duel. As noone attempted to stop this, being powerful as he was, Sunset accepted.  
The duel started with him simply posturing, taunting her with small spells and minor telekinesis. As she got angrier, he simply retaliated by knocking her back down, thinking she would soon get to embarrassed and surrender. Sunset persisted, however, and after she managed to cut one of his legs with her horn, drawing first blood, he used his magic to throw her back, and began summoning lightning around his horn, planning to kill her quickly and make an example.  
Sunset looked up from the ground to see him lowering his horn, crackling magic setting her fur on end. He set off the spell and in a split second of panic Sunset threw her head back, casting a momentary shield and deflecting the worst of the lightning. In anger the noble reared back to cast another spell, and with adrenaline now coursing through her Sunset pushed herself forward, lunging and with only thoughts of winning on her mind, a poweful solar flare of magic, like a spear of flames, rushed with her and caught him in the center of his chest. He fell dead, and Sunset stood the victor, with an image of the Sun shining on her flank.  
Only a few minutes of stunned silence from the town later, Celestia descended to the scene having heard the call for the Suns witness, and saw the young Sunset standing alone over a burnt body. First She felt shock, then understanding, and then hope, as She was looking at the first wielder of solar magic since the last of Her champions was killed with Luna’s betrayal.  
With this hope, Celestia took the young foal to Canterlot to study the arts of combat and ancient solar magic, and to train her to become the next Champion in time for Nightmare Moons return.  
However, Celestia was careless, believing in her hope that Sunset would understand the importance of what she learned and mistook the foals eagerness for admiration and loyalty. It wasn’t long after that Sunset started sneaking into the corners of the library, hungry for knowlegde and unsatisfied with Celestias slow and cautious teaching. She soon found a way into the forbidden wings, opening old scrolls depicting celestial armies and ancient battles, sketches of the heart of the badlands and lost castles, and accounts of the Sun Champions, asmall group of powerful Equestians that followed only the word of the Sun Sister. Sent out to bring Her Light into the far reaches of the world in gleaming golden armor with Her magic branded and weaved into their beings. Sunset believed the records to mean that she would be bound to Celestias will entirely, only recognized as Her servant and without free will.  
It was also this time in the forbidden section that Sunsets presence attracted a demon, who took the opportunity of the feelings of betrayal and anger to feed them, whispering to Sunset about a mirror with incredible power hidden in the palace.
 
Blah Blah Blah beginning of EQG Sunset confronted Celestia, got pissed, ran through the portal and went to highschool.
 
After Twilights ascension Sunset made it back to Equestria and stole the Crown, with the vague plan (supplied by the demon) being to use it’s magic to transform the school into powerful beings, and using the mirror as a way to ambush and infultrate the palace from the inside, the goal being to take the throne from Celestia and have the elements ascend her. It obviously failed, with the elements following Twilights will, turning the magic on Sunset and purging the demon. Sitting alone in the crater of the aftermath, alone in her thoughts for the first time in years and looking up at the princess standing victorious above her, she realized that she had fallen to the cruelty that almost killed her as a child. Horrified at this understanding, she surrendered, accepting the execution she knew would be waiting for her in Equestria.  
Twilight, however, saw the horror and regret in Sunsets face, and believed she deserved a second chance. So Twilight pardonned her crimes, and let her stay in the world of humans as a form of parole with the only caviat that should the princess need a warrior, Sunset would be bound to her and not Celestia.  
It would take a few years for Twilight to call on Sunsets help, with war fast approaching after Celestias fading. Twilight only calls on her for short amounts of time, prefering for her to stay with her friends in the human world. When she IS called to fight however, she’s put directly in charge of a small squadron to take on magical threats wherever need be, and even sometimes is tasked with training frontline unicorn soldiers on combatitive magic. She becomes well known as a powerful and respected Archmage and is often called the Champion of Dusk, and despite that fame having previosly been her dream, she now prefers the quieter life with her friends on the other side.