Disclaimer: I don't own any of this, I don't profit of this. Harry Potter, MLP and Fate Stay Night are protected by the law and registered by their original Authors.
Sun Hide, Moon Show
Universe transport is generally fatal or at least painful.
This is because other worlds have different wavelength, going from a universe to a parallel one is the least painful since the difference in the wavelength of the energy is minimum, resulting on a mid discomfort making it easier for the host universe to recon the intruder (traveler).
The longer the travel the different wavelength it has, meaning the intruder is seen like an abnormality by that universe, just like Gaia rejects magic or magical changes; this has been known as the abnormality effect, in which the universe recognize the external object and takes different actions, the first is to suppress it till it is crushed by the aura in the universe, completely eliminating the intruding abnormality, yet it can take time like a poison as the wavelength becomes more and more toxic with the difference or it finish it out immediately if the difference is too big. This has the involuntary effect to flood it with the aura so if it survives it changes the wavelength to a compatible number giving the intruder the chance to not affect the environment.
Yet if the world is more mana friendly, the universe takes other route, consisting in eliminating the intruder by what mages refer as "translating" where instead of attacking the wavelength of its aura, it changes it to the point it is so similar to the ones in the universe, specifically the dominant species, reconstructing its body so that the aura produced is compatible, if a wurm from a different dimension was transported to Gaia, then Alaya would give it a human body; of course it can be painful as it is only the physical changes, the mind and conscious of the soul remain the same even if the vessel is different.
Kaleidoscopic is a Chaotic magic that transverse the rules, as a true magic they can't be replicated and have the benefit of giving the user a way to ignore the worlds will, or in others way bend the rules of the universe without it becoming hostile. It just drops the user in a portal and then surrounds them in with an aura compatible.
"Damm you Zerletch" Shirou screamed as he fell beyond the clouds, with a baby on his hands and a backpack, as he was literally dropped in a new world. "I should have asked help from the enforcers"
The portal transported them to a forest, 8,000 ft in the air, so it was understandable that the free fall wasn't well received. As the two fell in a certain death, Shirou refused to admit defeat, and especially he refused to fail this child, checking the area as he descended he managed to catch the form of a castle, yet too far to become of use at the moment.
As the floor drew near, the second magus killer could devise the landing zone was near a couple of cliffs, maybe he could, yes he wanted to try that some time ago. Reinforcing his clothes and the basket in order to resist the force he tried to maneuver directly in the cliff, before extending his hand in the air.
TRACE ON
Immediately green magic formed the blue prints as a weapon was recreated. In Shirou's hand appeared a decorated Staff, Ruyi Jingu Bang, a B+ Rank noble phantasm used by Sun Wukong, also known as the heroic spirit Son Goku. While not very costly, its abilities of the Steel Rod were very taxing as you need to upkeep them.
"EXTEND"
The monkey staff suddenly grows both sides till it covered enough territory, so the instant he reached the abyss both ends the staff locked on the edges of the cliff near a rope and wood bridge; the same staff bended slowly bleeding the force of their fall before it recovered its strait form.
Smiling to himself, Shirou looks own to the baby on the basket on his other hand to see baby Harry happily clapping and laughing. (Remember babies need a lot of stimuli and he used a toy broom on his house on canon). With a mental command he started to extend one side of the staff while shrinking the other in a good impression of a bandolier, as he gets closer to a side of the abyss he starts to wonder why the od drain he feels is barely the minimum.
Once he managed to reach and climbed the other side, a bit difficult with the baby basket, Shirou glanced back and forth as the Noble phantasm returned to its original size, giving a sigh he tries to find a path "you know Harry, maybe there is some one in the castle that can help us" talking to the baby he follows the path that comes from the bridge as the sun finally raised completely. It was mid-step when he glanced the baby on his arms.
"Carrying you like this will become troublesome" maybe, using a bit from Archer's memories he device something more practical.
TRACE ON
In his hands a Kangaroo carrier appeared, big enough to fit Harry, quickly he strapped the child to his chest and adjusted his backpack to make it more comfortable, as the child looked at him with his wide open green eyes. Shirou tried to think on what he could do at the moment, using the monkey staff as a waving stick in order to advance on the path in the forest, for it seemed to be made for smaller travelers.
"Agaga"
THUNK
GRRRrrr
As Harry giggled a bit, Shirou was distracted enough to overextend his arm more than necessary, making the pole arm hit something solid instead of separating branches. The growl wasn't good news, so he slowly moved his line of sigh till he could clearly see the creature he accidentally strikes.
Wood, the first thing he managed to see was wood, as the figure resembled a dog made out of wood, just like the one in front of the Edelfet house, just this one wasn't a statue as it appeared to be growling and baring its teeth on his direction.
GRRRRR
Four more growls made him aware he was surrounded by this wood wolf, and with only a staff to fight. A quick reinforcement gave him a boost to jump quickly to the left, preventing him to be blindsided by one of the creature. Choosing to fight them he dropped the staff and extended his arms with his hands opened, it was only then he felt a weight shift on his chest and he remembered the baby. Normally Shirou is suicidal enough to fight against the odds, but at this moment he would not put in danger the child he decided to rescue. Shirou needed to help him, but how could he…
The CASTLE
Before any of the wolfs managed to jump at him, Shirou used a Prana Burst to dash in the direction of the castle, of course the castle filled with people that can help him, people that can watch the baby while he fight to make sure little Harry remains safe.
A quick tracing and Kanshou appeared on his left hand, only to impact in the mouth of a timber wolf after a small swing. The situation was getting bad if they were already trailing him. Instead of confronting them Shirou jumped to a tree and taking the benefits of reinforcement he managed to kick the solid wood and propel himself higher, this time dissolving his black sword as he charged more prana to his circuits.
Harry started to cry as the magical energy formed a dirk like weapon, the aura it expelled making the air uncomfortable to the baby. A quick swing and the pack met Pyriburst, a dagger that was made in the clock tower by accident in trying to recreate the flame sword Flamberg, yet what they got was a dagger that once in contact with mana and air it will wildly combust, but if it was dropped the strike would release the prana stored and explode.
As the explosion rang in the middle of the wolf group and the pack busted into flames Shirou held Harry tighter as he dashed direct to the self imposed safe heaven as the sky darkened.
Majestic was the sight in front of him, or it would be were not for the derelict and jungle invaded image the castle ruins held. For the sight, it seems like the castle was devoid of life, well not life at is it, just abandoned.
With a sigh Shirou shifted the now hungry Harry a bit in order to enter; at least it was a roof so he can't be too picky. On the other hand didn't Rin told him about undead guardians and mechanical golems protecting ancient ruins so maybe it would not be too safe to enter, any thought was stopped as a thunder filled the area, making Harry cry as the water started to pour.
Having the decision taken from him, Shirou decided to enter the ruins in order to seek shelter, trying to calm Harry.
He spent several minutes dodging corridors to discover this place was really abandoned, he continued exploring till he entered a well illuminated chamber, tall dome ceiling, stained windows deciphering equine animals in different colors, all in all a cozy looking place including the fact that he could no longer her the rain or thunder from outside, yet what caught his attention was that in the middle of the room was a weird statue that supported six glowing stones.
A sense of foreboding filled him as he walked near the statue, yet he ignored it, Harry was hungry and that was a priority so taking his back pack the Emiya teen started to remove his supplies till he found a couple of apples, a container and a bottle of water.
He smirked as he remembered how to make apple mush, the smirk turned a soft smile while preparing it as he remembered why he knew how to make it.
Taiga at her 17 broke her bike, and instead of asking her family for money she decided to win it herself, so she tried different Jobs.
Painting; she had to paint a fence, yet the whole block was crimson for at least a month
Food delivery; only one store asked her, because she actually beat the client with Torashinai when they didn't give her a tip 1
And last Baby sitting; it took at least 10 minutes before she declared herself unfit, another 5 minutes to travel and kidnap Shirou from his own home; and another 5 minutes to convince him to cook for her and the child. Another 3 to call Sakura for help and while it took 1 for her to come. In the end he learned how to cook for babies, Taiga was fired when the parents saw the other 2 kids and the memory of Sakura Smiling while holding the baby and trying to call him papa was completely repressed.
He was waked from his memories as he felt the texture of the mush, only to discover one thing, THERE WAS NO SPOON.
Hours later
It was almost noon time as the darkness of the storm settled. Luckily baby Harry was fast sleep after eating the whole mush, and Shirou managed to create a semi-refuge near the statue, for it gave the basic needs. Like protection from the elements thanks to the rooms ceiling along with a comfort zone to watch for threats as there was only 2 entrances and one was locked.
The shelter was also surrounded by a couple of boundary fields that detect hostile intent giving them enough protection for the time being, or at least till they need to move in search for civilization.
Shirou tried to meditate about the situation and why Zelretch transported him here. "Ok I asked for it, but not in that way". He mussed silently as a strange smell filled his nostrils.
"Strange it smells almost like magic, but a different" he commented to no one, but as he closed his eyes to discover the source of the magical smell, Rin once said he was a magihound to be able to smell magic, he missed when the stone spheres glowed brightly, then the glow faded.
BOOOM
A loud explosion made him stand up battle ready, both hands to the side in case of tracing, as he saw some light enter from the windows as the storm dissipated, and what could be described as a wave of rainbow entered by the walls and collided with the stones making the glow brighter.
His nose told him that was a magic wave, of different classes, or at least that was before the stones shined and the Pain started.
Magic likes to follow certain guidelines, and the stones being the Solidified Elements of Harmony weren't the exception.
These guidelines were the most basic of intent, conservation and adjustability according to the caster.
Intent being the creation of harmony, in other words if there is an anomaly to change it till it reach perfect harmony or balance; they are now focused on the two beings near. Two living life forms surrounded by Chaos magic, filled with and unnatural energy different from the hearth and themselves but most importantly an imbalanced essence; so as the intent from their creation they tried to fix it healing the damage or what could be considered the break of balance in other words the Horocrux in Harry's Head and Archers arm attached to Shirou.
The magic rainbow easily dispelled the chaos surrounding them, with the barrier that prevented the magic in the air to classify them as intruders the pain started on both but at the same time gave the elements another object to change and is to make them balanced with the magic in the world.
Magic will always look for the route that uses less energy in order to conserve balance (unless is chaos magic) and reach its objective, so when the energy overload them the Horocrux in Harry dissolved into his soul, but the Essence of what was identified as the "flight of death" couldn't resist the harmonic magic and got dissolved before it could corrupt the baby leaving the rest that was part of some magic abilities and some information as the other was too damaged to be used and was discarded; but it was one of those abilities that was left behind that reacted in resonance.
Shape shifting 2
The ability unlocked was easier than to bombard the beings with magic till they were one with the ambient and was more balanced that to forcibly morph them, so it mixed the energies of the baby, the adult and its own to activate the change into a more harmonic change.
The baby began to gain a equine form, but as the magic was detected a small horn sprouted on its forehead, the other was no more difficult, as the magic was internalized on its own like the earth race, the horn wasn't necessary.
So by magic Harry Potter became a unicorn foal while Shirou was mistaken for an Earth pony as his reinforcement was very similar to how the earth species used their magic, the problem was that the caster and unwilling and unrelenting 10 year old Pegasus his body shrunk to the appropriated age. (3)
Numbness
Numbness and Exhaustion
Shirou opened his eyes, as he felt like the time he used his circuits too much and lost the sense of half his body. He could see the darkness being pierced by the full moon as it bathed the room.
Groggily standing up, Shirou examined his body and found no missing parts, when he remembered the nightmare he had the young magus turned everywhere in panic in search for Harry only to find him sleeping peacefully. Shaking his pain aside Shirou grabbed a bottle of water an took a sip.
Him a horse, but not only a horse but a pony like the plushy he found on Rin's room and that he jokingly said she was only studying under Zelretch to create a portal that would lead her to them before she shot him on the back with a Gandr Curse4, what a strange dream.
Unable to rest, the young magus examined his hand, no the hand of who he would never become in the future, Archer or better yet Emiya, the one that followed his fathers footsteps till the end and regretted it. It was dark but he could see some difference in the arm, it was no longer that dark tan, thinking it was only a trick of light he took the place near Baby Harry and keep guard for the night.
It wasn't long before the light of the moon disappeared as the sky took an rosy and orange tint; the magus smiled as he could continue with the plan to travel till he found people, and a better place to take care of Harry.
Since the numbness disappeared Shirou quickly packed an retraced another Kangaroo bag, thinking the last one he made would disappear soon.
It was then that he despaired, once the light o the sun touched his hand he could feel the magic inside him along with other energy shift and his hand morphed. No pain came but the panic raised as his hand turned into a Hoof then his arms filled with fur, the change came natural, once the light reached harry the baby changed too.
Where once a baby was, now was a brown foal with a horn and black hair.
Where once was a teen, now lay a tan earth colt, with red hair and the red cloth he received from archer adjusted to his frame.
It was then when he knew, that nightmare was no more than reality, and only the moon will know of their real forms.
In that Instant Shirou Cursed, he cursed the clock tower, he Cursed the Wizard Marshall and he Cursed the root, He cursed the Root mostly.
Ok another chapter a go, I know this are a bit short, but then again I try to make something new, hope you like it, as for the transformation, many do it and yea It can be a bit cliché, but this type is new.
The reason for the sun and moon is that since Luna was banished her connections to the elements were cut of, so only Celestia had the connection at the moment, before the others gained them. So every time they are hit by sun light they change to pony form, and under the moonlight their true self shall be revealed.
(1) I can actually see Taiga doing that
(2) Way better to use than Animagus than to just transform them, it gives more possibilities.
(3) A cookie to determine the event
(4) Never tease a tsundere, much less a magical tsundere and worse if it was a closet pegasister magical tsundere.