Komatsu Sadamune

A vassal of the Kuki clan during the Sengoku period.
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Komatsu Sadamune approached Kuki Yoshitaka with some intriguing information: three mermaids were said to be living on Taishima under the protection of some of the island's residents even though the island was thought to be uninhabited.
Sadamune urged Yoshitaka to take advantage of this, suggesting that if he possessed such power, and invincible army would be his to command.
Yoshitaka gave the order for Sadamune to prepare a military invasion of Taishima.
Sadamune immediately set sail for Taishima at the head of a large army. In the blink of an eye, the once peaceful and beautiful landscape of Taishima was transformed into a nightmare.
The islanders, guardians of the cared land, desperately fought back, but they stood no chance against seasoned soldiers hardened by years of battle on the frtonlines, and one after another they fell before the invaders. Sadamune ordered that even women and children be cut down without mercy, and soon the waters around Taishima ran red with blood.
Finally, they discovered a mermaid in hiding by the name of Hayase and captured her, torturing her to death.
Eventually, he found a deep cave in the interior of the island, which housed a shrine and a door with a power seal. Convinced this had to be the entrance to the "sacred land" —the home of the mermaids— Sadamune gathered all his remaining gunpowder and artillery and unleashed it on the door, destroying both it and the shrine in the process.
The next moment, a violent tremor wracked the entire island, and the skies above were engulfed by a storm. Taishima sank beneath the waves, and thus Taishima and the sacred land —along with Sadamune and his army— lost to the sea in an instant.

Spoiler

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Sadamune was in fact Taira no Tomoshige, who had eaten mermaid flesh and gained unnatural longevity.
Unable to let go of his dream of reviving the Taira clan even after 400 years, he caught wind of rumors of mermaids being worshipped on Taishima, and infiltrated the Kuki navy under the name Komatsu Sadamune, plotting to uncover the sacred land. However, his rash and impulsive destruction of the sacred land ended up triggering the Curse of the Raging Waves, and his plan ultimately ended in failure.
Although caught up in the sinking of Taishima, Tomoshige could not die, and was swept away by the sea to an unknown fate. He never directly met Princess Hatsune, and as such the two remained unaware of each other's existence.