Celestia accidentally broke day

Luna rushed through the halls of Canterlot. She was wearing as much clothing as she could to keep the rays of the Sun from touching her. All she needed was to be sure her sister was okay. She had to be okay. She came across some of the flesh monsters begging her to join them, the same as the others, and blasted them in defiance. She made her way around another corner and was finally at her sister's chambers. Luna saw that there was a magical barrier around the room, and not only that, but the same creatures trying to break in with magic. The realization they could still use it only made her more frantic. The hoard finally took notice of Luna, but it was already too late as they were blasted away by a surge of alicorn magic. Luna ran up to the wall of the barrier and pounded on it.

"TIA! Are you okay? Let me in!"

Luna waited for a response, and seconds later heard the familiar, distraught, voice of her sister.

"GO AWAY, YOU MONSTERS! HOW DARE YOU USE MY SISTER'S VOICE TO TRICK ME INTO COMING OUTSIDE!”

A pause.

“Wait m-my sister's voice...t-then, that means…"

Luna could hear her sister begin to sob from inside, and slammed her hoof down harder on the door in response.

"Faust damn it, Celestia! Open up this barrier right now. We don't have much time!"

Luna looked around the hallway to see if any more of those things there. As she waited, she heard screaming coming from the other side. She readied a spell, horn glowing. She watched as a unicorn guard turned the corner and came running down the hallway. She began to loosen her stance, but would soon regain it as she saw a hoard of monsters chasing the guard.

"You have got to be shitting me. Tia, you better open up RIGHT NOW!"

Luna took a defensive stance and began to blast the flesh monsters that were chasing the guardstallion. With some of them gone, the guard made a beeline for Luna. He skidded to a stop next to her and tried to catch his breath, nearly collapsing from exhaustion and relief.

"I assume you are not only exhausted physically, but magically as well?"

"Y-Yes, your highness."

Luna leaned down as she transferred some of her own magic to the guard. The stallion seemed instantly rejuvenated. They both began to shoot at another group much bigger than the one prior. Their efforts this time were fruitless, as the surviving creatures began to combine with themselves, making sure that they never lost numbers and becoming stronger in the process. As the hoard approached, Luna heard a click. She looked to her right only to see her sister staring back at her, then past her down the hall.

"BARRIER, NOW!"

Celestia lit up her horn as fast as she could as she took and down the barrier, Luna quickly running in to the room. Luna looked back to see if the guard also made it in, but he was still outside and as pale as a ghost. The guard looked to Luna as and reached a hoof out for her. Realizing the situation, Luna quickly moved back to the door and closed it shut behind her, putting her body weight up against it, so panicked that she didn’t even think of locking it first.

"TIA, THE BARRIER NOW!"

"WHAT ABOUT THE GUAR–"

"TIA!" Luna screamed.

Celestia looked away as she re-cast the spell. They both felt the rumble of some of the hoard passing them, some hitting against the magical wall hard. Luna took her time to recover as she looked her sister in the eye for the first time.

"W-why?"

"’Why’ what?"

"WHY DID YOU LEAVE HIM OUT THERE TO DIE?!"

"FAUST DAMN IT CELESTIA! I KNEW HE WAS GONE BY THE LOOK ON HIS FACE!” she wobbled on her hooves, shock and horror coming on all at once, “I couldn’t…the hoard was too fast for him to get in.."

Celestia was silent.

"Tia, please tell me. What in Equestria is going on with the Sun? The world is in chaos. It was fine just an hour or two ago but now..."

Celestia bit her lip as one of her front hooves began to shake.

"It's all my fault."

Luna's eyes went wide, and she took a nervous step back.

"I had hoped you had nothing to do with this Tia…but now I have to ask the question. W-what did you do, Tia?"

Celestia began to choke on her words as she tried to address her sister. "L-Luna I–"

A loud crash was heard as Celestia's window exploded. The cursed Sunshine came pouring through. The both of them backed up into the darkest corner and watched in horror as a flesh monster floated inside, one larger and more grotesque than the others, a mess of flesh and fur and limbs. Celestia's eyes turned to pinpricks as Luna took a defensive stance, charging up as much magic as she could.

"No…t-they got to her. What have I done?"

The creature was massive. The only distinct thing about it was the protruding horns and wings among the grotesque mess of parts. It could have well been Cadence, but any doubt was shattered as it spoke.

"Hello, mentor."

Celestia could hear Twilight's voice as the dominant speaker, but she could hear others, primarily the rest of the Elements. All combined together into this horror.

"I have come to show my wonderful mentor the purity of her Sun. Come with me, Celestia. I’m still your student, right? You trust me…"

Luna and Celestia said nothing, the former’s horn now nearly as bright as the light coming in through the window.

The horrid thing that was once Twilight Sparkle shrugged. “So be it. Then we’ll do this the hard way, then.”

Celestia averted her eyes. She couldn’t bear to look as Luna charged. There was a flash, a scream, and then–