The Aftermath (A WarAU)

Even after Fluttershy’s disappearance, Twilight tried her best to only disarm her opponent and rely on diplomacy to end the war. She organised hundreds of meetings between her and the enemy leader but even if the leader turned up they were unable to compromise. She was gradually worn down by the impasse. Her friends gradually turned against her as she refused to see their perspective, Twilight’s infallible optimism sickened them, they thought her intelligence would make her less naive. The 16th battle is when she changed her whole demeanor, the last battle at Canterlot was her undoing. The specific moment can be linked to the blast. Unknown to all was the origin of said explosion, those who had survived it had conflicting stories on its appearance, radius and destruction. What is known is no magical shield could’ve stopped it and it was the most likely catalyst for the collapse of the castle which had ultimately ended the war.
If you can’t tell already, the blast wasn’t an enemy attack, as believed by all.
Twilight was at the end of her tether. She’d lost all her friends to disloyalty or death, everything she’d tried to prevent further violence had been in vain. A beam of sunlight and a jet of the darkest black shot in opposite directions from the castle balcony. Twilight was informed by Shining Armor that the Princesses had fled, Celestia to the sun and Luna had willingly returned to the moon. As Cadence had abdicated as it was against her charge to fight, Twilight was now the Queen of Equestria. Twilight’s world shattered. She was abandoned by her two mentors, left to take the fall for a war that appeared at a stalemate. While processing this information, she was fighting off vicious attacks from all angles. Her brother did his best to defend her as she ingested the news that she was in charge, hoping it would push her to do something to give her forces the upper hand. And indeed, she did.

It wasn’t clear, even to her, if her magical blast of incomprehensible heat was intentional or a release of pressure and stress she couldn’t control. She never took credit for the blast which turned her brother and friend’s legs into a smear on the castle courtyard. She didn’t fight any further after the blast, it wasn’t necessary. Her army fought valiantly, so much so that even though they were outnumbered 1 to 10, it wasn’t clear who would’ve won had the castle not collapsed.

Upon the wars end, she was promptly crowned in a ceremony where none of her loved ones attended aside from the recently widowed Cadance. Twilight was not the mare she was before. She didn’t care for friendship now, seeing it as not a weakness or strength, but an unnecessary addition to one’s life. She was now totally indifferent to the lives of others. She developed some cruel tendencies such as hunting animals for sport (she enjoyed displaying her mounted trophies above her throne and study), restoring capital punishment and installing several puppet leaders to keep all to her liking. When the princess of the sun and the princess of the night returned, they were quickly manipulated into remaining subservient to their new Queen who threatened them with execution for their desertion during Equestria’s darkest hour. Her tyrannical reign would see no end within the lifetime of her former friends who she made no effort to contact or defend from the harsh ruling she’d imposed.