"I'm sorry, you know I can't do that." Celestia said to a privileged noble. She was holding her morning court as she normally does while regaining her strength from raising the sun.
Celestia hadn't detected the buildup of energy, whoever was channeling it had the skill to mask it even from her. The discharge, however, was impossible to miss. An immensely powerful orb of dark energy was launched at her nearly at the speed of light, ripping through the chamber doors and leaving behind a trail of energy resembling the night sky.
It would only take a fraction of a second for the spell bomb to reach its target. There wasn't nearly enough time to raise even a basic shield, let alone one powerful enough to block a spell of this magnitude. Attempting to dodge would likewise be a futile gesture. She would have been struck down in a single blow, if not for one thing. Celestia, being the goddess of the sun was also the goddesses of light, unparalleled in her mastery of teleportation.
Celestia reflexively teleported out of the way of the projectile. It attempted to home in on the sun goddesses, arcing in the direction where she now stood. However its sheer speed made it impossible to perform such an extreme maneuver and, as a result, impacted the magically reinforced stone walls instead.
Darkness filled the chamber as the bomb exploded and the chamber doors where blasted open in a shower of splinters around the spot where the dark spell had penetrated. The darkness receded back into the alicorn-sized crater left behind by the attack. As light returned to each noble and politician, some would scream out and cower in fear, others would come to their senses immediately and attempt to flee, though most took varied amounts of time to recover from what just happened.
A cloud of midnight blue smoke blew past the broken doors with purpose and into the chamber. The smoke swirled and formed a body as large Celestia, a large horn and a pair of wings forming soon after. The pair of large wings then flared out, blowing the smoke away to reveal an armored Luna.
The lunar princess wasted no time and immediately took an aggressive stance to once again attack her shocked sister. Her horn flared a midnight blue as she charged up to cast a less potent, but still deadly spell. Teleporting out of the way again would be both easier and less tiresome than attempting to bock it but while other ponies remained in the chamber she wouldn't risk having them get killed in the crossfire.
Luna cast her spell at Celestia but this time she had been ready and raised a golden shield to block it. The darkness bomb impacted against it and dispersed into a cloud of night sky. Without lowering her shield, Celestia turned and called out to the ponies. "Run my subjects! Run!" That snapped the ones that hadn't fled yet out of their shocked stupor. They stumbled onto their hooves to immediately run towards the blasted exit.
Celestia slowly circled around her sister, keeping herself in-between Luna and the innocent nobles. Luna regularly cast her dark spells at her counterpart while occasionally targeting ponies outside of Celestia's protection forcing her to teleport to block the attack. Luna took every opportunity to wear out her sister before the real battle began.
Eventually all the nobles where guided to relative safety and Celestia now dodged Luna's ranged attacks with ease. Luna could run after them if she wanted to, use Celestia's kindness as her own downfall. Though simply defeating her sister wouldn't be enough, she'd need the acceptance of her subjects. There would be no benefit in traumatizing the ponies she will soon any further than she already had.
Celestia knew that battles cannot be won with defense alone, now it was her turn to attack. She teleported out of the way of another of Luna's spells and began to cast her own. Celestia harnessed the power of the sun itself to conjure a flaming ball of pure fire and light. She had been worn out a great deal from protecting those ponies so it wasn't as powerful as it could have been but it was at least on par with her sister's darkness bombs. Celestia screamed as she cast the spell for the first time in thousands of years.
Celestia's flames raced at Luna at blinding speeds. The beauty of her spell is that it doesn't even need to hit the target to destroy them. The heat of the spell was so intense that it could cause severe burns just from passing by them.
Luna didn't try to dodge or even raise her own shield to defend herself. Instead, as the miniature sun raced towards her, her whole body turned into midnight blue smoke. The spell harmlessly passed though her smoke body to turn a decent portion of the nearby stone wall into molten rock. The smoke cloud reverted back to flesh and Luna countered with her darkness bomb as soon as she was able. Celestia once again dodged with a teleport and once again cast her flames only to have Luna repeat the pattern.
They went on like this for several minutes, both unable to gain an advantage over the other. This could have gone on for days (provided that they had breaks for raising and lowering their respective celestial bodies), clearly ranged combat wasn't an option.
Since Luna had made the first move, Celestia figured she should make the first in melee combat. She focused a moment while Luna prepared for whatever her sister would throw her way. After a moment Celestia simultaneously teleported behind Luna and, with her magic, conjured up a massive golden blade large enough to slice an alicorn clean in half.
Her vision around the blade was warped as it radiated waves of heat. Celestia couldn't help but scream as she swung the golden blade at her sister as it left behind a streak of golden light. Right before the blade struck, Luna turned into smoke far faster than she'd demonstrated before. Turning her flesh into smoke is far easier and can be done much more quickly than doing the same for clothing or armor. As Luna's body became smoke her armor was left in the air to fall. Celestia's blade sliced through the smoke and and armor cutting both in half. The dark polished armor clattered to the floor in two pieces, glowing red hot from where the blade had sliced.
The smoke, which had maintained its alicorn shape, reverted back to an unarmored, flesh Luna. She bucked Celestia in her chest which made two large dents in her own armor and caused her to stagger backwards. At the same time Luna created her own sword which gently glowed with the beauty and splendor of her moon.
In one swift motion Luna blindly swung her sword where she knew Celestia stood as she turned to face her. She saw the blade swing towards the still staggering Celestia's head. It would be a grizzly, but quick, death. As soon as the blade penetrated her skin Celestia reflexively teleported out of its path before it could go any deeper. She could feel the stinging as blood ran down her cheek and stained her pristine white coat. She touched her hoof to her cheek, not to wipe it away, but to feel the strange wetness. She looked at the blood-her blood- and was reminded of something that had been forgotten. Fear.
Fear of death. Fear of the prospect that even gods can bleed. Fear of what would happen to Equestria should she die. That fear then became anger. Anger she focused on the one who had reminded her of these things. She growled at her cold, calculating sister and then screamed as she exploded in anger.
This was exactly what Luna wanted. She had anticipated that her sister would eventually become enraged and hold nothing back. The hotter a fire burns the more quickly the fuel runs out; it's surviving the initial fury that be challenging. But that suited Luna just fine. Though her smoke power is incredibly useful, one of its flaws is that it can't move about very quickly. Its true strength then lies in defensive combat. Even against her full fury Luna had the upper hand.
Celestia released a battle cry as she charged towards Luna, who was waiting in a defensive stance. The fight played out dozens of times in Luna’s head as she tried to figure out every possible move her sister could make and a counter to each. Celestia got within swords reach and raised the sword to bring it down on Luna, who had raised her own sword to block it. As Celestia’s blade swung downwards she teleported behind Luna while still swinging it downwards.
Luna had decided that whenever her sister teleported out of sight it would be safest to turn to smoke until she could tell where Celestia was. Her caution payed off as Celestia’s blade harmlessly passed right through her and embedded into the stone floor, leaving it red hot and molten around the blade.
Luna reverted back and bucked her sister as hard and fast as she could just as before. Her hooves passed right through thin air as she realized her sister had reacted much faster than last time and had teleported back in front of her for another attack.
Celestia swung her blade sideways as fast as she could, aimed right at Luna’s neck. Her blade once again passed through smoke and as soon as it had cleared it reverted back, Luna ready with her counter. Celestia wouldn’t let Luna have her way so as soon as she reverted Celestia headbutted her face faster than any other attack, horns scraping hard against each other from the maneuver.
Luna staggered, taking a few steps backwards as Celestia used her wings to jump backwards onto the elevated platform that held her throne. Luna stared up at her, gritting her teeth as blood trickled from her broken snout. She had expected Celestia’s anger to make her careless, make mistakes, not to help her focus and become an even more vicious fighter.
Celestia slowly landed next to her throne with all the majesty one would expect of her. She took an aggressive stance once more and called out to Luna, “I know not why you have attacked me sister, but rest assured I will not allow you to strike me down. Please, lets stop this senseless fighting before it tears both of us apart.”
Luna took a moment to consider her sister’s proposal. Would she really allow this to go unanswered? Would Celestia risk letting her go, or would she use the elements of harmony and attempt to imprison her once again? No, Luna had gone too far to back down now. She had to finish this now or Celestia would regain her strength and gather her assets. She wouldn’t stand a chance then.
Luna gave her answer in the form of a darkness bomb which left Celestia’s throne a blackened pile of rubble. Celestia’s expression saddened at the realization that there could be no peaceful resolution. She had no more words for her sister.
The two stared each other down, each waiting for the other to make the first move. Luna would only have the advantage if Celestia attacked first so to encourage her she would cast weak darkness bombs as Celestia teleported around the chamber to dodge them. Eventually Celestia decided she had to attack one way or another and teleported right next to Luna to do just that. Luna jumped back out of reach of the blade as it swung at her and cast a darkness bomb at her sister. Celestia had begun to feel the fatigue from the constant fighting. She was now limited in how quickly she could teleport so she had no choice but to attempt to block it. With no time to raise a shield she instead moved her sword to block the spell with its flat side.
The sword itself had a lot of energy put into it; more than enough to cancel out the energy of the spell. The bomb impacted against the sword. The part of the bomb that directly hit it was dissipated right away but the bomb was wider than the sword itself and a fraction of the spell managed to get past it to graze Celestia’s hide. The sides of her hide had been blackened and burned by the dark energy and as soon as she was able she teleported away to recover. She tried to teleport out of the chamber so she’d have more time but the magically treated stone was resistant to magic and Luna’s spell had somehow drained her energy enough that she wasn’t able to go through. Instead she ended up behind one of the columns, hidden away from Luna’s line of sight.
Luna spread her wings and flew a few dozen yards into the air to search for her injured sister. She spotted Celestia leaning against one of the columns tired and panting in pain from the magical burns. Luna gave her no quarter and flew at her at top speed and swung her moon blade in an arc as she drew near.
Celestia raised her own blade blade to block it but it struck with such force she was barely able to keep her magical grip on the weapon. Luna pushed harder and harder against her blade and Celestia pushed back. This was now a battle of raw strength but Celestia was nearly burned out. In the end she knew she wouldn’t have enough magic left to overpower her sister. Celestia’s face strained as she attempted to teleport away but blocking the initial strike had been too draining and she didn’t have any energy left.
Luna continued to push against Celestia’s blade with her own and eventually she felt Celestia’s magic waver. With one last push Celestia’s grip on her blade was lost and it slowly dispersed into golden particles as it was knocked away. Luna adjusted her strike at the last second so that it wouldn’t harm Celestia, she didn’t want to kill her just yet. Luna’s blade passed through the column, slicing it clean in half and leaving it a pile of rubble as it fell to one side.
Luna’s blade hovered over the burned out and defenseless Celestia and they looked each other in the eyes. Tears streamed down Celestia’s eyes, “Please sister...Luna...don’t do this,” she pleaded, eyes fearful and desperate. Luna closed her own eyes and her breathing grew heavy as they also began to tear. Finally this moment has arrived. But could she go through with it? Could she actually kill her sister? She knew she had to; there was no other way. Luna’s voice trembled as she spoke, “I’m sorry Tia...please forgive me.”
Celestia cried out in pain as the moon blade sunk deep into her and, starting from where the blade entered, her body begun to turn to stone. As more and more of Celestia turned to stone, Luna used her magic to absorb her life essence, ensuring that she couldn't one day return. She tried to remove the blade from her now stone sister but she couldn’t summon up the strength to do so. It would forever serve as a reminder of what she did.
Luna cried and sobbed at the loss of her sister. It was all over. Celestia was dead.
EDIT: I was thinking about making this a tie, kinda like what happened between Goku and Vegeta in DBZ (if u have any idea what I'm talking about, good for you) but at the end I liked Luna more so I let her win. To all those people that think Celestia would win without a doubt id just like to point out that she was defeated by the power of love