Umibōzu are yōukai from Japanese folklore that inhabit the sea around Japan. They are known for causing storms on calm days whenever a ship comes across them, before either destroying the ship or demanding a barrel from the sailor and drowning them when they comply. In the latter case, it is possible to escape by giving the Umibōzu a bottomless barrel and escaping while it is confused. Umibōzu are supposedly the spirits of priests who were murdered and thrown into the sea, and who have now returned to seek vengeance against all who come their way.
According to folklore, Umibōzu suddenly appear if sailors come across the area they inhabit, and turn the calm sea stormy. Many legends also state that Umibōzu appear in the sea at the end of the month, during which time no-one can go out to sea. However, some legends tell that if you do, the Umibōzu will not immediately kill you when they appear, instead asking "Am I fearsome?", at which point the sailor should reply either "There is nothing as fearsome as navigating through the whole world" or "I am afraid. Please help", to which the Umibōzu will reply "You should not be going to sea at the end of the month" before vanishing. If you answer something else, the Umibōzu will destroy the ship.
Other tales tell of Umibōzu shapeshifting into beautiful women and tricking humans into engaging in swimming contests with them. Those who accept will be dragged down to the depths by the Umibōzu and devoured. Umibōzu would also sometimes shapeshift into blind men known as Zatō and go onto land to hunt and kill human women. Umibōzu also occasionally rose out of the sea and went onto land in their true forms and return to the sea after a three-day period during which they would take any children who ventured outside.
In some legends the Japanese gods controlled an Umibōzu known as Kuro Nyūdō who would rise and pursue ships with women on board, which angered the gods. Kuro Nyūdō would pursue the offending ship until the women was thrown overboard, allowing Kuro Nyūdō to devour the woman. If the woman was not thrown overboard, Kuro Nyūdō would continue to chase the boat until it eventually caught up and destroyed the ship.
Umi Oshō was an Umibōzu that took the form of a human with a tear from mouth to ear. Upon finding a real human, it would laugh hideously and devour them. Umi Oshō was the most feared of all the Umibōzu, as a natural disaster was sure to follow if it appeared.
Another Umibōzu, known as Fune Nyūdō, would appear in the sea with no eyes, nose, or limbs. If one was seen, it would not attack the ship unless it heard a crewmember wondering what it was, in which case it would immediately sink the ship and kill all on board.