Hayase

The curse echo of The Curse of the Forsaken Fish.
Curse: Kills any human submerged in the waters of Ise Bay by gradually turning them into a fish starting from their feet.
Its curse bearer is Arnav Barnum.

A curse born from the final moments of Hayase, a mermaid brutally tortured to death by humans on Taishima around 400 years ago.
The excruciating pain suffered during the transformation often results in the victim dying before the metamorphosis is complete.
The bodies of those who succumb to the curse decay almost instantly, shriveling up into desiccated, fibrous masses. The curse can also be triggered remotely, provided that the curse bearer knows their victim's exact location.
Mermaids inherently possess the ability to turn humans into fish at will. The Curse of the Forsaken Fish is the result of that ability being turned into a conditional curse and sealed within a curse stone.
* * *
During the Invasion of Taishima, Hayase stayed behind on the island as bait to allow Sobae, Nagame, Princess Hatsune, and a man named Somekawa to escape.
Komatsu Sadamune's forces discovered Hayase in hiding and captured her. Sadamunde demanded that she make his troops immortal, but the mermaid staunchly refused — even as Sadamune turned to torture, inflicting unspeakable pain with every refusal. The torture escalated, until eventually even her supernatural nature was not enough to withstand the torment.
On the brink of death and resigned to her fate, Hayase gave a final terrible cry, overflowing with resentment and anger as she breathed her last.
Sadamune ordered his men to take the flesh from her body, but as they moved to do so, a strange phenomenon began to occur — all the soldiers who approached her body began to transform into fish, their bodies decaying rapidly as the transformation took hold. Hayase had cast a curse upon them with her dying breath.

Spoiler

* * *
The two remaining mermaids — Nagame and Sobae— were left unable to return home. Nagame seized the opportunity to travel the world, a journey that would span the next 400 years. The more serious Sobae was deeply affected by Hayase's sacrifice; mourning her loss and filled with longing to return home to Ryugu, she spent her days hidden in the depths of Ise Bay, weeping bitterly.