The term 'Underworld Ruler' refers to two characters from Greek mythology: PLUTO and HADES. PLUTO, also known as HADES, was the ruler of the underworld in Greek mythology. He was the son of Cronus and Rhea and the brother of Zeus, Poseidon, and Demeter. HADES was the god of the dead and the ruler of the underworld. He was often depicted with a helmet and a three-headed dog. HADES was also known for his ability to make people disappear into the underworld.