Umiana
Her actions led to the loss of more than ten million lives. Smaller kingdoms on the coastline of the oceans ceased to exist as if they never were, while more powerful nations had shielded themselves from the wave with the help of powerful Martial Artists, and esoteric technology derived from esoteric sources that were nothing short of national treasures.
She never ceased to practice her technique, keeping the hole open for nearly the next twenty years. The surrounding nations had already adapted to the elevated sea levels by the time her training phase had been completed. When she threatened to leave, the coastline nations realized that her leaving would leave the whole filled up once more, which could cause a disaster on par with the original disaster that she had caused.