Perhaps 'guarding' was not the correct word for what we do here tonight Luna mused as she hopped from one enormous branch to another. The tree rivaled most mountains in scale, and would be impossible to climb (especially for those who have only their hooves to move them about) if it truly existed. But here in this dream of carefree exhilaration, one could leap great distances with no effort at all, skipping among the leaves and feeling the wind in one's mane. Luna couldn't remember if she'd ever felt this way, but Applebloom's joy was infectious. They had trotted across dreams, pausing in any that Applebloom thought would be fun. And what fun this is!
"This is amazin' Luna!! Everypony's dreams are so different, and we get to visit all the best- woah!" The filly stumbled as the tree shook, nearly falling from her perch. Luckily a branch bent of it's own volition, bracing her, cushioning her with it's mass of leaves.
"Careful, young one. Though the tree seems to be looking out for you, it is time to carry on from this place. Though dawn is not yet near, this pony will be rising early. We feel him waking" she explained, gesturing to the stallion bounding through the leaves nearby.
Applebloom looked mildly alarmed, "what happens if we don't get out before he does?"
"Not to worry. When a dreamer is ready to wake, the dream collapses into the raw dreamscape, with enough force to send all inside back to the waking world. It can be unpleasant for those far from their bodies though, so it is preferable to travel back by linking dreams."
"Well I guess we better get on to the next dream then. We've still got time to play some more before we have to head back to our bodies, right Princess Luna?"
"Indeed we do! Onward!" Luna pulled in a nearby dream and leapt from the branch through the border, followed a moment after by Applebloom.
Dream after dream, most quickly passed through, being of little interest to the excitable filly.
A dream of bubbles. Thick foamy bubbles covering the ground, bubbles of all sizes and colours floating about. Bubbles you could ride, bubbles you could push your way inside to turn the world around you blue or green or pink. There for a time, then on to the next.
A dream of sitting by the hearth, feeling warm and safe and surrounded by loved ones. So peaceful...
"Luna, what happened??" came the unusually high-pitched shout from behind her.
Turning, she immediately recognized the trouble. Here was Applebloom, reduced somewhat in size.
"I feel so little and weak!!" The filly was distressed, but not nearly as much as Luna would have expected.
"Worry not, Applebloom. It seems we have arrived in the dream of your sister. She still thinks of you as small and needing her protection, and so you have become small while in her dream. I have learned to resist such things, but you have never ventured outside your own dreams. All will be well when we leave."
Applebloom inhaled deeply, then exhaled and calmed herself. She looked at her sister, sitting among the dream of her family. "I guess I can forgive you sis. I know it's hard to remember I'm not so little anymore."
Applejack smiled at her, and Luna knew somewhere Applejack's sleeping form smiled as well, dreaming of her little sister.
Minor crisis over, the two carried on. There would be time enough to find one more dream that Applebloom approved of.
At first glance, the desert didn't seem like the sort of place a filly would enjoy. They sat in the shadow of a great sandstone sculpture of some creature that perhaps never was. Pyramids rose up from the sand nearby as well, but the architecture was not what held the young one's attention.
Applebloom had done plenty of running around in the other dreams, and had decided to end the night with a show. Even in her dreams, the sapphire pegasus with the rainbow mane was quite the showoff. She and a tan pegasus in a pith helmet were flying fanciful patterns between the heads of a Hydra. Luna would feel bad for the confused creature, knocking it's heads together, far from the swampland it calls home. But of course it was only imagined.
"Yeehaw!! Go on, get 'im Rainbow Dash!" the filly whooped, clapping her hooves together in delight.
Luna's attention was half on the battle before them, half on Applebloom herself. Perhaps this need not be just this one night.
Luna smiled, her attention all on Applebloom. But the filly's own smile faltered. Luna frowned, following the young pony's gaze back to the Hydra. The helmeted pegasus was lifting herself up from the sand, looking as though she had fallen hard. Rainbow Dash's flight had become more frantic as the Hydra became quicker and more coordinated.
Something was amiss. Luna reached out with senses she had allowed to become dull through a night of merry-making. There, just breaching this dream, was something all too familiar. Nightmare energy. Not the nightmare of a troubled pony's sleep, but a sentient force that fed on fear and anger. She had spent a thousand years wrapped in it's embrace, becoming one with it as Nightmare Moon.
She felt that it was weak, a mere fragment of Nightmare energy, appearing only as a dark smoke. Celestia's pupil and her friends had ensured that it was no longer the danger it once was. But she had entered the dreamscape nearly exhausted. And she had foolishly brought along a child! She didn't think she had the strength to defeat it if she had to protect Applebloom as well. What of Rainbow Dash? No, it knew it posed little danger to a dreamer in her own dream, it was snaking it's way through the air toward her and her young charge. She must return to Applebloom's dream so that she at least was safe. Perhaps then she may even be able to draw it back to her own dream where she had the advantage.
All this passed through Luna's mind in a moment. "Run!" she shouted, though she was unsure now that they would make it the few steps to the border of the dream. The Nightmare had become pointed, lancing toward them now that it felt her fear for Applebloom. She herded the filly forward, trying to put herself between her and danger.
But Applebloom glanced over her shoulder and saw what was coming. Her eyes widened with realization that something was about to harm the Princess. "LUNA!!" she screamed, not in panic, but with some other emotion.
Luna felt the both of them being thrown forward as the sand beneath them heaved. Stumbling and trying to watch behind her at the same time, she saw the massive sandstone wall slam up out of the ground, throwing sand high into the sky. She felt the Nightmare come up short on the other side, apparently unable to penetrate this sudden construct. Taking advantage of this momentary pause, the wall tilted, unfolded, became a pyramid around the creature. Images of monsters being defeated, trapped were etching themselves into the stone, forming haphazardly all across it's surface. The purpose of this edifice was made clear: it existed to keep the Nightmare within.
A few moments of confusion and gratefulness passed as the two of them watched the pyramid sink into the sand, further entombing it's prisoner. Breathing heavily, Luna finally took charge of the situation "Applebloom, I can feel the Nightmare in that pyramid working it's way out. It poses no danger to Rainbow Dash, but we must use this time to put distance between it and ourselves. Let us return to your dream. There you may awaken and be safe."
Merely nodding, Applebloom followed Luna out of the dream.