The term 'River of the Underworld, in Myth' is often referred to as the Styx, which is a four-letter word. In Greek mythology, the Styx is a river that separates the world of the living from the world of the dead. It is also known as the river of hatred and is said to be a fast-flowing river filled with the souls of the dead. The gods of Olympus were said to have sworn oaths on the river, and it was believed that those who crossed the river would be cursed.