Leviathan was the name given by Earthlings to these types of sea monsters, which could be found throughout the oceans of Otherworld, delineating their territories everywhere. No one had the chance to see what they truly looked like, only glimpsing the sight of them churning through the ocean from a great distance.
Leviathans were naturally aggressive and would attack any ship that entered their territory, so the people of Otherworld generally only dared to navigate coastal areas. The water in these regions wasn't deep, prohibiting Leviathans from dwelling there and hence allowing the people of Otherworld to venture into the water.
It was because of the fear of Leviathans that the people of Otherworld directly called the ocean the Forbidden Sea. When Earthlings named the ocean, they simply adopted the term "Forbidden Sea."
