In an alternate timeline, Twilight did not save Luna from herself. She and many others perished before Nightmare Moon. In her bitterness and grief Celestia fell to her own inner darkness and became Daybreaker.
***
Wildfires tore through the Everfree Forest, the results of the calamitous battle being fought. Both the sun and the moon hung in the sky, night and day splitting the skyline. Daybreaker and Nightmare Moon were settling their grudges.
Like a streaking comet, Nightmare Moon slammed into the scorched earth. She dug a furrow through the ground as she planted her hooves to arrest her momentum. The last bits of her breastplate dropped to the ground as scorched and melted slag. She grasped at her side where a cauterized wound cracked and bled.
Daybreaker landed nearby, a small blastwave sending dust in all directions. She smirked at her sister and looked at the sky. Two-thirds of the sky now yielded to the sun. She sauntered closer to her wounded sibling, oozing arrogance and power.
"Your finished little sister!" Daybreaker cackled. She looked appreciatively at her sisters heaving boobs and shapely thighs. "I’ll be sure to give your body the worship it deserves."
Nightmare Moon only smirked defiantly at her sister. "Arrogant foal, you've already succumbed to.."
A fiery blast struck Nightmare Moon, burning her to ash. Or was that smoke?
Daybreaker frowned as she looked around her. She now stood in the restored ruins of the Castle of the Two Sisters. Gone were her wildfires and a quick look through the windows revealed a sky almost completely veiled by night. She spun in a fury, launching balls of fire.
"Where are you coward?!" Daybreaker screamed. The straps on her armour snapped and her breastplate clattered to the floor, followed by her cod piece. Her flimsy golden dress followed shortly, with razor cuts in the straps.
"Tis cunning to let your foe wear herself out before landing the killing blow." Nightmare Moon's sultry voice whispered into Daybreaker's ear.
Daybreaker spun again launching a red hot beam that cut clean through the stone walls. She panted from her exhaustion, her glorious boobs bouncing from the exertion. Daybreaker was drained from her phantom battle. "I'll crush you when I find- HHHHHHYYYYAAAAAARRRGGGGHHH!"
A shard of black crystal burst out between Daybreaker's tits, splattering her blood and gore on the walls.
Daybreaker staggered and looked down at the fatal wound. She moaned and tried to grasp the magic missile. Her hands reached a third of the way and Daybreaker dropped to her knees.
Nightmare Moon formed from mist infrobt of her defeated sister. She knelt before Daybreaker and tilted the mare's chin up. "I win!" She kissed her sister's gasping mouth.
Daybreaker's horn sparked with fitful magic. Her hands dropped limply to her side's and her hips bucked, Nightmare Moon was stroking her damp cunt.
With her free hand Nightmare Moon grabbed Daybreaker's horn and with a visious twist, snapped it of.
Daybreaker moaned into her sister's mouth, pain unimaginable coursing through her body. Her hips bucked again and she let out a fountain of urine into Nightmare Moon's hand. The last of her strength ebbed away and her eyes dropped. Daybreaker died in her twisted sister's embrace, her heart torn from her chest by the crystal shard.
Nightmare Moon broke her kiss and looked at the slack expression on Daybreaker's face. She snorted and lit her horn, casting a teleportation spell. Both Alicorn mares appeared on the huge bed in Nightmare Moon's bedchambers.
"On the floor where you belong whore!" Nightmare Moon hissed, kicking her sister with a shod hoof. As her sisters corpse rolled of the bed a telekinesis spell removed Nightmare Moon's breastplate and cod piece.
Daybreaker landed on her back, her lengthy legs stretching over her head. This humiliating pose exposed everything; her boobs, ass and cunt reflecting in a mirror. Her broken horn tip clattered on the floor from Nightmare Moon's casual toss.
Stripped to her helm and ornate horseshoes, Nightmare Moon made herself comfy on the edge of her bed. She lit her horn again and a magical two-ended phalus was conjured at Daybreaker's ponut.
Daybreaker's ponut resisted the magic cock for a moment before being stretched wide. The other end of the cock pushed against Nightmare Moon's own ponut, being accepted with a little more difficulty.
Nightmare Moon let out an erotic groan as she readied her body. Her anus clenched on the magical dick, pushing it further into Daybreaker's ass. "I'm going to enjoy every moment of this dear sister." With that she began thrusting herself against the magical dildo while manipulating it back and forth with her horn.
The night will last forever.