The creature known as the guardian of the underworld in Greek mythology is Charon. He is the ferryman who transports souls across the river Styx to the realm of the dead. Without a proper burial or payment, souls are denied passage, which reflects the ancient Greek beliefs regarding death and the afterlife. Charon's role underscores the importance of rituals and honoring the dead in ancient Greek culture, as it was believed that failure to do so condemned souls to wander in limbo.