Princess Luna stood on the shores of a world she had never known, bearing witness to an apocalypse she could scarcely comprehend, with both sides led by minds she found hauntingly familiar.
The world was Dominaria. That much was clear from the war-cries of the... humans. These were humans. She recognized them from the world on the other side of the mirror, and from Twilight's disturbingly detailed reports. And those were...
Well, for the moment the only obvious answer was 'monsters'. Most of them had a form that seemed vaguely inspired by the humans, but they had no proper muscles or flesh to speak of, and the mechanisms that made them move seemed to have been both built and grown. Their movements were jerky and unpredictable up close, but from a distance they seemed to move more like a wave crashing over the landscape. It was very disconcerting.
Then one of the creatures lunged at her, and the feeling of the blade passing through her face was upsetting on a number of levels.
First, because the last thing anypony wants to experience is a buzzsaw ripping through their gray matter, especially when it's attached to such a gigantic pile of smelly pus and recycled flesh. Second, because the only sensations that could be felt in any particular dream were those that the dreamer had felt in the real world, and that wasn't the sort of thing a pony was supposed to be able to walk away from. Third, the monster had aimed directly for her own face.
That wasn't supposed to be possible.
"Possible? HAH!"
The monster standing in front of Luna exploded in a burst of fire, and she tried not to be too surprised when bits of its corpse landed on her and stuck there. A new creature was standing before her, its claws still glowing with red magic, and its lips parting in a very predatory smile. It put her in mind of a panther, except for a few key differences; it stood like a human, wore jewels like a king, and was glowing with more magic than an alicorn.
"Who are you to say what is possible and what is not? Is this your dream? Is this your world? No!" The panther flicked its wrist, sending a bolt of fire flying through the air. Luna ducked out of the way, despite part of her brain still insisting that the magic couldn't hurt her. Then she turned to see where the fire had gone, just in time to watch another one of the ghastly creatures explode.
"You... are not the dreamer." Luna didn't know whether to phrase it as a question.
"Of course not. I've been dead for hundreds of years, and I never even visited your lovely little plane." There was a touch of regret to the panther's voice, but it felt ill-suited to him, as though tacked on by the dreamer's thoughts. "I am simply one more voice in her head. The name is Lord Windgrace." The panther extended a paw. "Pleased to meet you, I'm sure."
"The pleasure's all mine," Luna assured him, her curiosity and mild horror briefly overcome by etiquette training. She put her hoof in his paw, and as she shook it she couldn't help but shiver at how warm it felt. "Now, would you mind explaining what is going on here?"
"Might as well," Lord Windgrace sighed with the resignation unique to those who have just realized their struggle is not real. "This is Dominaria, days, perhaps weeks after the Rathi overlay began. The Phyrexians are pouring out in full force, and all who can fight for their world and their right to survive. Urza has very nearly gone mad once again, there are whispers of traitors on both sides, Bo Levar won't stop trying to peddle Golgari rot-cigars to anyone with breathing room, and I have to hope the crew of the Weatherlight is doing something effective with their time. In the meantime, mana lines have been going insane, the lords of Dominaria are spending far too much time trying to turn the apocalypse into political maneuvering, planar portals have been popping in and out of existence faster than any of us can blink, and there's about a billion more things going on that are all a little fuzzy and you have no idea what I'm talking about, do you?"
Luna, who had been been nodding blankly throughout most of the panther's speech, nodded harder at those last few words. Windgrace sighed, then started laughing. One of the monsters took the opportunity to stab him through the chest, but that only made him laugh more.
"Didn't you get the memo, Phyrexian pig?" Lord Windgrace's head turned in ways that should not have been physically possible. "We are but dreams, like unto dust should the dreamer wish it. But dream or not, one thing will always ring true!"
The world flashed red, and Luna felt everything burn away. When the flames cleared, Lord Windgrace stood alone, and the wound in his chest was nowhere to be seen. He grinned at Luna, and looked around at the landscape of ash. Humans and monsters alike had been burned to cinders, but the sight of all those dead did nothing to faze the panther. On the horizon, there were figures, no larger than humans but radiating power the likes of which Luna had never felt in Equestria.
"They can bring all their world against us, but we need only bring nine." Lord Windgrace turned back to Luna, then spread his arms wide. "This is Dominaria. These are the first days of an invasion from another world. They were not the last. But at the end of it all, we did win. Not in this precise way," he kicked a newly-made pile of ash for emphasis, "but we did win. After all, how else would you be seeing this now, if she had not survived?"
The bone and ash behind Luna crunched under hooves, and she became aware of a fully conscious mind standing behind her. "Howdy-do, princess. Fancy seeing you here."
A few billion questions flew through Luna's mind, but she already knew which one to open on. "The avatars of your dreamscape are aware of me. How is that possible?"
"There's a whole load more universes out there than you think, missy. And most of 'em ain't all that nice. You spend twenty years wrestling with a psychic dragon, you learn a few tricks to protect the secrets you want kept secret."
Luna turned to face the pony whose dream she had invaded, and refused to let her avatar show fear. The pony was drenched in blood, festooned with weapons, and far too calm for somepony who had just been stood in the middle of a swarm of death. But the worst part was the pony's eyes, older in cruelty and knowledge than the eyes of herself, her sister or even Discord.
"I can help," she said weakly, defaulting to her old script. "I can help you make these nightmares go away."
"What nightmares?" The blood-drenched pony looked around at the ashen husk of Dominaria, then smiled and waved at Lord Windgrace. "This is one of the best dreams I've had in twenty years!"
In her chambers, Princess Luna keeps a list. It is a list of everypony whose dreams she has access to, with notes on sleep patterns, recommendations for how to deal with nightmares, and sometimes just little bits of trivia that surprised her the first time she stumbled across them. But perhaps most important is the security ranking, the space in everypony's entry where Luna puts down how dangerous they are and how closely they should be monitored. Most of these entries are blank or written in pencil, as ponies can change depending on a great number of factors.
When she finished her conversation that night and awoke in her chambers, the first thing she did was turn to Granny Smith's entry, pull out a pen and fill in the blank space next to her security ranking. The second thing she did was go into her private bathroom to throw up.