Many people believe that the story of Kirishima started with the Wadanojin's crew stranding, and while it's true to some extent, it doesn't capture the whole picture From one side of the story, it all began with a clam that could breathe illusions. That talent was versatile, and the clam, being prideful as it was, started to float about its miraculous power: it would often pull pranks on weaker fish by making them see the predators far away. Their power would only grow from here, to the point of being able to replicate the sea capital itself to its minute details - naturally the clam would get cocky.
It began to pose as a small dragon, demanding small and big sea creatures to bow to it - calling them the "Illusory Fog Dragon". That clam would also commit serial forgeries of the valuable treasures and basically create a false existence for themselves. One of a noble, powerful dragon that can change the items' forms and not of an ordinary lowly clam with illusory breath. They convinced everyone below them of that existence and they themselves also began losing themselves in a process, feeling as if they were untouchable.
However such days would end when the real dragons were involved. They exposed the facade that the clam kept up, and while the clam's illusions were strong, at the end of the day they were mere illusions, and clam could not make up for them - clams are naturally very weak fighters. The dragons were ready to execute the clam for its blasphemous attitude, but it hastily escaped the capital. It didn't look back even for a second, that's how much it feared death.
Eventually their running led them to a deserted island. It was relatively big and it had some foliage growing on it to hide from the pursuers. The clam swam up to the island and decided to take a rest, feeling that they could finally catch a break, but soon the waves crashed and they were submerged once again. The island was alive. It was a giant Akaei. The ray was ready to devour the clam, but it pleaded "O great sire, please do not eat me! I can prove myself a great help for hunting! My illusions will lure the boats towards your way if you spare me!". The ray was intrigued "Then prove it to me. Recreate a Ryuuguu for me and I'll spare you."
It was a tall order, but the clam eventually recreated the capital with their illusions. The ray expected them to fail spectacularly, but was taken aghast by how faithful the illusions were to the real deal. The ray spared the clam and allowed it to ride its back to lure the boats towards them.
That ray turned out to be an exile, as it took a liking to eating human flesh, or any kind of flesh in general, which allowed them to grow abnormally big. The ray would've been executed by now, but it was too huge for the authorities, so they just prohibited the ray from ever entering the capital. It didn't mind it however and was content eating everything that went their way.
The ray didn't expect the clam's illusions to work and thought of them as a silly partner who also committed a taboo, but to its shock, the illusions worked. Many foolish sailors would fall into the trap of visiting the legendary sea capital, only to be eaten by a ray. It seemed like they were made for each other, so their bond grew closer. The clam was forever thankful to the ray and was enamored by its sheer strength, while ray was intrigued by clam's powers. The ray would've been fine without the clam, but after many years of solitude, they too seemed at least some sort of companionship, even if it's built on mutual support.
Their devilry didn't become an urban legend because no one could live to tell the tale, and both of them liked it that way. Clam due to fear of being caught and ray due to his averseness to humans. And both didn't want to get hunted down, so there's that. It was a perfect relationship. But every good time has to come to an end.
The ray was sleeping, becoming so gargantuan that even when sleeping it resembles a small island. Thinking it would be easy to kill and eat some humans as food like by themselves, the clam started to lure the strandees again. But this time... It was different.
When the clam saw the captain, it froze. They couldn't believe their eyes. The captain was two meters tall, tall as a Wani in human disguise, and branded a fine sword, the metal from which it was made could only be obtained in the sea and the technique used to forge this weapon was only taught by sea dwellers of the Ryuuguu. The clam tried waking up the ray, but he was a heavy sleeper, so he wouldn't budge.
The captain and his crew of humans and youkai alike kept exploring the "island" they got stranded in. These humans and youkai were strong too, the clam would never stand a chance, so it looked over them for an opening. Eventually, through illusions, the captain approached the centre of an island and plunged his sword into the ground - killing the ray. The clam couldn't do anything. No matter how much they tried to wake up their beloved friend, no matter how much they tried to mislead the crew - it was over. And by some sort of miracle, the ground beneath them started to flourish with greenery. That captain was celebrated as a hero and the crew started to build their new civilization on this island. Clam hated that. It started to cause even more deathly illusions as an act of revenge, but the dragon retaliated by bringing forth the shrine priests - his descendants. They would make a charm that would repel the illusionary mist that permeated the island, and when the clam would retaliate physically - they would end up badly injured, even against their so-called nemesis which forced them into silence. The clam was all alone.
Days turned to weeks. Weeks turn to months. Months into years. Years into decades.
Decades into centuries. The "dragon", as clam claimed, was deified and now became a rare sight and clam could only watch his "homevillage" from afar to bide their time for a pipe dream revenge
Clam gave up on creating illusions by now, preferring to be a recluse, fending off for themselves. There were many more outsiders coming in - some left, some stayed. One of them however, was odd even by clam's standards. That person arrived alone on the boat.
The key word "arrived", no one arrives on this island on purpose; usually the people who arrive here are strandees that don't know any better.
That person had this uncanny aura to them. He concealed his identity with a mask, had a ton of luck charms, and overall he had dark attire. There was certainly something wrong with him. That person couldn't be a human, yet even by youkai standards - that person was an oddity, something above low-level yokai.
When he expresses emotion, there is a distinct hollow quality to it, as if that person was just pretending to feel it. His movements are stilted, as if he was controlled by a mechanism. And their voice. It is clear that person is a male specimen, but while he charms humans with his eloquent charisma, to the clam it stroked primal fear. Perhaps this is a youkai who detests youkai, yet loves humans.
Perhaps this is a youkai who detests youkai, yet loves humans.
Clam was especially careful, keeping their distance away from that mysterious youkai, however it seemed like it was set to settle down like every other foreigner due to charming a daughter of a chieftain. It seemed like this village, the one that keeps that dragon enshrined, will be completely unapproachable from now on.
One night however, it saw that same youkai running away in a hurry from that village; its face scarred and clothes tattered. "Well, good riddance" thought the clam carelessly and smoked their pipe in relief. It checked on a shore where the youkai's boat was and sure enough it was gone and far away it could see a figure becoming smaller and smaller. It didn't pay attention to that oddity, but little did that clam knew, that this small event from a perspective of this lonesome vagabond would kickstart a chain of events that would their pipe dream of revenge a reality.
A year passed and sure enough, it saw a priest of that dragon carrying a horned baby.
The priest was ready to kill it, but they couldn't do it out of its goodness of their heart and decided to keep it. "It's probably that youkai's spawn, that guy had his pants down."
Remarked clam to itself. The priest loved the baby and took care of it as their own and soon that horned baby grew up to be a child. It was timid and always kept filing down their horns despite how painful the process is. It also concealed their face as it looked just inhuman enough to be noticeable. The clam pitied the child for being forced to hide their true identity like them, but nothing more, it seemed like it was going to inherit the title of a priest.
What clam didn't know however, is that the child shouldn't exist for a reason. The child was soon found out to be alive all this time and the villagers were enraged. They chased down the priest who protected the child and executed them by hanging. The clam was shocked. These folk seemed to be untouchable for the clam, but now... They see their corpse, hanging from the tree, and being used as a target to throw rocks at. The villagers also destroyed the shrine in their blind rage, but clam wasn't happy about it. With the savages like these, clam isn't going to be satisfied with the revenge as these imbeciles would target them as well: plus, the dragon and his envoys weren't their sole target, it wanted revenge on every single one of its inhabitants for desecrating his friend's corpse. And so it went into hiding, continuing their way as a vagabond.
And soon the first opportunity finally presented itself. The god felt enraged by the display of blasphemy and caused a drought. The villagers tried to repent by building new shrines, but the damage was already done. The clam then would create illusions of phantom rain, feeding them false hope and laughing in glee when they saw their pathetic mugs wail in despair. While to the clam it was just a cruel prank, for the villagers, it was a sign that "THEY" had resurfaced with the vengeance.
And way later, the true showstopper appeared. The recently executed priest became the most threatening vengeful spirit on this whole island who went on a massive killing spree and it gave a clam an idea. It began conjuring the illusions of that spirit or even masquerading as them. This exploitative nature of the clam became beneficial to both parties. The ghost became even more feared, and thus more powerful thanks to clam's mimicry - and since it was so feared, the clam wouldn't worry about someone finding out about them.
That ghost became known as the Tatarigami of Kirishima and it was thanks to that clam that the Tatarigami became so infamous. It was thanks to that clam that the Tatarigami could conjure fog thanks to the power of the belief; it used to repel it when it was alive, but now it uses it as a cloak to keep their appearance hidden for their target. It was thanks to that clam that the Tatarigami could morph into hideous forms because of how the inhabitants perceived the ghost. In a sense, the clam WAS the biggest contributor to Tatarigami's imagery thanks to the power of superstition. If you believe something is going to happen without any explanation, then it'll happen. Tatarigami was already strong, but it was the clam that got them the traits most associated with them, specifically the ghastly fog that surrounded them.
And in a sense... Perhaps they're the true vengeful spirit of Kirishima. A clam, which was forsaken by the sea dwellers and abused by the inhabitants of their desecrated friend's corpse, is acting out their revenge on everyone who wronged them, puppeteering the poor priest who was unjustly executed. Perhaps if it wasn't for the clam, the vengeful spirit would've been pacified much sooner.