Four princesses enter through the ancient door, the hallway providing the only light in the room. Celestia and Luna set to igniting torches and activating illumination crystals while Dawn and Twilight both ambled in awe, staring all around them at the scene. Bookcases fifteen meters tall lined the room. Magical instruments of varying degrees of both size and complexity lay around the rotunda’s tables and hanging displays, each one sans labels. Alchemic experiments, or potion production as far as any of them could tell, sat in varying degrees of completion, if one overlooked that nearly every liquid component had long-since dried to a husk, and all fuel sources were spent. An incomplete manuscript still was splayed over the lectern with an inkwell, desiccated by the unrelenting march of time, beside it. Another set of double doors sat on the far side.

“Whoa...,” breathed Dawn and Twilight in unison.

Wow. Not even the Starswirl Museum had this many original specimens.

“I see you didn’t change a thing,” Luna chuckled, shaking her head.

Celestia gave her sister a sad smile. “Nothing, except reinforce the enchantments. This room would have been dilapidated if I hadn’t.”

Not that I’ll ever get to visit the Starswirl Museum again...still can’t believe, and won’t believe, the gods decided to destroy us. I have to get home. I have to bring it all back somehow.

The royal sisters stopped in front of the other set of double doors. Luna turned to her sister. “Could you ever open these doors?”

“No...,” Celestia trailed off as she turned toward Dawn. “But I think we know somepony who can.”

Luna and Twilight also looked to the interdimensional visitor. Dawn met each of their gazes, and sighed. She powered up her horn. “Alrighty then, let me see what I can do with this.”

I sure hope you have some secret in here that’ll help.

The doors opened without any struggle. Within was a small chamber, with shorter, not as full bookshelves reaching the not-so-high ceiling. Instruments on non-book shelves were smaller, far more intricate, also without label. Many of these gadgets were still whirring, among which a few that had blinking red lights. The double doors at the other end were of a slightly different architecture, one that made Dawn gasp.

Well, that dispels all remaining doubts. Definitely the same Starswirl.

“Dawn...what is it?”

Twilight’s question shook Dawn from her from the trance. She looked at the three other princesses’ expectant faces, and sighed, “This door jamb looks like it’s from Cavalon. Like most of the things I’ve seen in this world, the difference between there and here is subtle. The flourishes on the molding are tighter, and turn the opposite direction of most of the door jambs here.”

“Yes, I noticed that immediately,” said Luna with a nod. “We should call for the archivists; these materials and devices certainly warrant careful study and preservation.”

Dawn pressed, “Of course, but after we look through them to see what can be done to help me, or if by one last miracle of his, he has a way to restore my world.”

Celestia said, “Naturally.”

The four princesses began thumbing through the ancient parchment. Most of them were diagrams, or nebulous notes about some intricate spell. They set the rolls they read in a common pile. After the eleventh scroll was set in the “read” pile, two archivists arrived. One quickly skimmed and catalogued the contents, then the other loaded them onto a cushioned, partitioned cart.

On her sixth roll, Dawn’s eyes widened.

To you who opened the door,

They call me Starswirl the Bearded. You may know me as “The Old Colt of Buckingham,” while those around you do not. That you were able to open the door is telling. You, too, are not from Equestria. And if you are reading this, trouble must have followed.

Equus and Equestria are competing worlds, just to exist. There are identical pieces on both sides, except for me. I have no counterpart. It seems I was destined to live in both Equus and Equestria, as if the gods themselves tapped me to either maintain the balance betwixt the two worlds, or to tip it. My magic’s wavelength is stable in both, while few (if indeed, any) others are as well, though this is particularly unlikely. I write this now for two reasons. One: if I was able to slip through the veil and end up here, one can easily conclude another may do the same, hopefully in time to prevent catastrophe. And Two: I realised I am dying, and will not be able to stave it off again.

Death isn’t the problem I face. Being interrupted by death is the problem; I am running out of time. I leave too many things unfinished: a perfected life-extending potion, the friendship spell, and most important, a functional dimensional stabilizing spell. My theories are sound (as evidenced by banishing a trio of Sirens from each world), but the complexities of affecting the whole of two worlds, not to mention sheer power output necessary, stymies my progress. For decades I’ve jumped between the two, gathering the data and testing the improved versions. I am close, so close I can all-but taste it. However, this current test still has two days before it completes, and I have another identical to it to running in Equus. It appears to be correct thus far, and if both tests are positive, then I will have it completed, and then the worlds can end their competition to simply be, let alone thrive.

If I have passed on, and the test has a positive result, I will need you to open the inner door; it opens directly to my identical lab in Castle Cavalon. You must summon Princess Celestia and Princess Luna to this room, and take the materials in here to Princess Galactia and Princess Soleia. Only the four princesses combined will have a sufficient power output to cast the spell, and at the proper wavelengths. Furthermore, only all four casting toward their counterparts through the open gate will stabilise both worlds. If not, the tipping of the balance will begin. As I said, I am the odd piece. On whichever side I die will gain the advantage. It will be slow, but will build up momentum over the course of a great many years, before the damage is likely to become visible.

If the test results are negative, somepony must continue my work, and the incomplete spell must not be cast. In its unperfected state, both worlds would begin their slow, agonizing collapse.

I pray this scroll is written in vain. You must hurry, if it is not too late already. In case it is...I have one last theory to test.

Starswirl of Buckingham
14th of March, Year 74 of the Age of Unity

He knew...Starswirl knew back then...why didn’t he say anything?

“Dawn?”

She felt Twilight’s hoof on her side. Dawn turned to see her royal peers and one of the archivists looking intently at her, though Twilight’s face was the most concerned. Dawn sighed, and held up the scroll. “He knew. All the way back then, he knew that the two worlds could not coexist. Starswirl knew one world would thrive at the expense of the other. He tried to find a way to reconcile them, and nearly had it shortly before he died.”

That would be why he stayed silent. It wasn’t the gods at all. It was him, because he could not right the balance before his death.

The others gasped. Celestia snatched up the scroll. Luna, Twilight, and both archivists rushed over to read over the monarch’s shoulders. Dawn frowned. As they read, she looked around. She scrunched her eyebrows and walked over to the door. She saw a bit of parchment, stuffed under the door by the hinge, where it was not visible outside of this small room. While the others were muttering amongst themselves, Dawn telekinetically pulled out this message, and opened it.

To you who opened the door,

The test appears to be positive, but my end is fast approaching. I no longer have enough magical strength to open the door, and my sight is going. Please find this room soon. My hope lies in that my assistant can find the castle physician or one of the princesses to get down here before I give up the ghost, and tell them how to open the door.

In the event this has not happened, and the collapse of Equus has begun accelerating, you may be able to halt or even restore the world...at least, if my calculations are correct. It, unfortunately, requires something drastic. You will need to find your Equestrian counterpart, and absorb them. This isn’t simply slaying them; this means you must extract their essence and magic, and draw those into yourself. Unsavoury, and unforgivable, if it weren’t for the host of lives it would save, or restore from the grave.

Regardless of circumstances, get the other documents to the princesses. Their combined powers will be necessary to stabilise both worlds.

I pray you can finish what I began. Godspeed.

Starswirl of Buckingham

Dawn was breathless. She could not inhale for a few moments, unnoticed by the five still engrossed in the other scroll. Looking around, she stashed the bit of parchment back whence it came.

He can’t possibly be serious. He wants me to kill Twilight? And that’s according to numbers he ran...when? What numbers, through what functions, based on what proof? Was he even cogent during this calculation? I can’t do that to her....

A tear ran down her cheek. Shaking her from the trance was Twilight’s voice: “That date, this was two days before he died. Did he get the results from his test?”

Luna’s voice was sadder. “From what we saw in The Orrery, either result would bear the same fruit now: nothing.”

“I hate to have to agree with you, sister, but I think you’re right,” said Celestia. “Without this other world, there is neither ‘Princess Galactia’ nor ‘Princess Soleia’ with whom to perform the spell. There’s no point in even attempting casting. Just the same, we should preserve what is here, as a last testament of his genius, and selflessness.”

No, Dawn. You have to try. It’s that or give up on Equus, your friends, and family, entirely.

Twilight sighed discontentedly. “That is so unfair to Dawn.”

Unfair is the right word, but more for you than me. There’s no way. She’s done nothing wrong.

There was a glint in Celestia’s eye for a split-second as she pressed, “Fair or not, we have to hurry if we’re to save either of you. Send word to Cadance that we may need her to transport Sunburst here; his expertise in magical theory may be necessary to decipher enough from these scrolls to find a way to stabilise both Twilight and Dawn.”

“Fair” has nothing to do with matters of life and death.

“Yes! He’s spectacular! Sunburst’s the best pony with magical theory since Starswirl himself trod Equestria! Nothing a good teleportation spell can’t fix!” beamed Twilight.

Good, an alternative. It’s not over yet.

Luna frowned. “If I may be so blunt, Twilight, you are unmatched in teleportation spells, but I doubt even you could successfully teleport two ponies that distance at the picture of health, let alone in your current state. They will need the train.”

“But that’s two days’ travel!” Twilight protested.

No, there isn’t an alternative, Dawn. It is over. You’re running away from the hard truth!

“Yeah, you’re right again, dear sister. I’m sorry, Twilight,” said Celestia. “I don’t know what we can do to sustain either or both of you, but I’m afraid we cannot send magical scrolls this old via dragonmail and expect them to survive. We’ll send for an emergency train, or maybe an airship, but at the rate you two are fading, I don’t think you’ll last for those two days.”

Twilight scoffed, “Surely there has to be something we can do!”

Celestia pursed her lips and closed her eyes. Luna looked like she going into mourning. Twilight kept looking between the two of them.

So this is it, huh? I have to kill her, and steal her essence?

Dawn began to sob quietly, but her tears came fast. Twilight turned to her, and put a hoof on Dawn’s shoulder. Twilight’s ears drooped. “I’m not giving up, Dawn. We’re gonna pull through this.”

You’re just making this harder....

Blubbering, Dawn slumped into a sitting position. She hid her eyes with a forehoof. Twilight rubbed at her shoulder. Dawn whimpered, “I’m sorry, Twilight...I’m so sorry....”

Twilight said, “It’s still not your fault.”

Makes me ill to just think about it, and I’m about to do that? What have I become...?

Dawn took the scrap under the door by her magic and tossed it toward Celestia. As the monarch unfolded the parchment in confusion, Dawn murmured, “I’m fresh out of options, Twilight...I’m sorry.”

What I would give for another way...but there it is. No sense in putting this off.

Twilight cocked her head to one side as Dawn got up, crying, “I don’t want to do this...I really don’t....”

Celestia gasped as she passed the scrap of parchment to Luna, and urged in a stern, mothering tone, “Dawn, don’t!”

Twilight started, “Huh? Celestia, what’s she talking ab—BAAA’AGGH!!”

Twilight turned in time to see the incoming magical strike. She moved to dodge, but the blue spell ray grazed her face, leaving a long and deep horizontal cut on her right cheek. She turned to see Dawn with her feet firmly planted, but her face was puffy-eyed, with tears so thick they were full streams running down her face, and rattling, sobbing breaths. “I’m s-s-sorry, Twilight. I-I-I am, but I can-n-not give up on my w-w-world.”

Twilight blinked in confusion, while Dawn charged up her horn, crying out loud.

Author's Note

Finally got back to writing a decent chunk at a time. This new job is...mind-numbing. It's like watching your creative abilities and drive atrophy right before you, while physically draining your body. Thus, the search for other work kept going. Wish me luck.

Then there's m' daughter. Once she figured out how to crawl forward, she began trying to pull to stand within a week and a half. Now she's starting to let go with one hand. I'm telling her to not grow up too fast on me, but it doesn't seem to be doing a lick of good....

The conclusion is next. Better have it out more quickly, too; I apologize for the unintentional hiatus.