In the unconscious plain, where all dreams take place, Luna trekked forward, comprehending a world to which she had only recently gained access. Her physical body sat crosslegged, meditating in the highest tower of the newly constructed Castle of the Two Sisters. At the extent of her vision, in this mysterious realm, she could sense something she only knew was not "right". Curious, she headed towards it, through the swamps of spousal arguments and over the jagged rocks of masonry inspirations.
She arrived at the source, the dream of an individual pony. It was larger and shimmered more brightly than any of the other dreams. The "odd"ness eminating from it gave her goosebumps. She tried to penitrate the membrane, and enter the dream, but it was far too strong for her. She understood that there must be a very peculiar mind that can house such a dream.
Luna touched the membrane and felt how strongly it adhered to itself, and only itself. She traced its rippling surface with a hoof, and tried to make her hoof the same as the membrane, shaping it to match the dream. It worked, the membrane sucked in Luna's hoof, and, then, the rest of her.
In the dream, a colt rested in a field after a hard days labor, watching the clouds drift by. Luna took the form of a crow, circling high above, just another unimportant detail of his dream.
The colt abruptly stood up. Though Luna wouldn't know, he had sensed her intrusion. The skies darkened. He could feel somepony else's presence. The field cracked apart and water surged up from underneath. The tumultuous current carried him away and he couldn't keep his head above water.
Luna tried to repress herself in his dream, interacting as mildly as possible. Still, she watched the pony get swept away and drowned by the water. His panic from drowning warped the entire dream, twisting her form as thunderstorms swept through the sky. The sense of oddness that originally attracted her to the dream propagated through the air.
She tried to disappear, to simply exit the dream, but found herself unable. She decided her only option was to calm the pony.
With the colt overwhelmed with the water, Luna discarded her disguise for her familiar form, herself as princess, complete with royal barding. Her eyes glowed from her powerful magic and she conjured a galleon to which she would ride to his rescue.
She took the helm, and steered the ship, with its billowing sails. She slowed it's descent before it plunged into the water with a giant splash.
She grabbed a rope, tied it into a lasso, and lassoed the drowning colt. She heaved, and pulled him out of the water and onto the deck.
The colt coughed up water and, in a moment, regained his senses. He looked up at her with gratitude. Then he realized she wasn't part of his dream.
He stared at her, and the dream ebbed with his fright. The concept of someone intruding here, the foreignness of it, he didn't know how to process it. Frozen, his jaw slowly opened and he screamed a scream that echoed throughout the sky and the water and the ship. It pierced Luna's ears, reverberating through and inside her.
She tried to escape the dream, but it was not a normal dream. It was far to strong to be simply exited. The power of the dream in this pony's mind was not something she could surmount. The colt's shock and fear shred her to piece, and ejected her from his dream with great force. In the castle's tower, she would be lying unconscious in a pool of her own vomit and urine. Her mind was trying to put reason behind what she had just experienced by creating, for her, a nightmare.