In Greek mythology, who guards the gates of the Underworld?

In Greek mythology, Cerberus, often called the "hound of Hades", is a multi-headed dog that guards the gates of the Underworld to prevent the dead from leaving. Cerberus was the offspring of the monsters Echidna and Typhon, and is usually described as having three heads, a serpent for a tail, and snakes protruding from multiple parts of his body. Cerberus is primarily known for his capture by Heracles, one of Heracles' twelve labours.

Descriptions of Cerberus vary, including the number of his heads. Cerberus was usually three-headed, though not always. Cerberus had several multi-headed relatives. His father was the multi snake-headed Typhon, and Cerberus was the brother of three other multi-headed monsters, the multi-snake-headed Lernaean Hydra; Orthrus, the two-headed dog who guarded the Cattle of Geryon; and the Chimera, who had three heads: that of a lion, a goat, and a snake. And, like these close relatives, Cerberus was, with only the rare iconographic exception, multi-headed.

The earliest mentions of Cerberus (c. 8th – 7th century BC) occur in Homer's Iliad and Odyssey, and Hesiod's Theogony. Homer does not name or describe Cerberus, but simply refers to Heracles being sent by Eurystheus to fetch the "hound of Hades", with Hermes and Athena as his guides, and, in a possible reference to Cerberus' capture, that Heracles shot Hades with an arrow.