The term 'Underworld Flower' is a reference to the flower of the Styx, which is a four-letter word. In Greek mythology, the Styx is a river that forms the boundary between the Earth and the Underworld. It is also known as the river of hatred and is said to be the most powerful of all rivers. The flower of the Styx is a symbol of death and is associated with the Underworld. It is believed to have magical properties and is often used in rituals and spells.