Cerberus was a three-headed dog that guarded the entrance of the Underworld, allowing the dead to enter but letting none out. He was the son of Typhon and Echidna. Apart from his three heads, he also had a serpent's tail, a mane of snakes and the claws of a lion.
Cerberus was the twelfth and final labour that was given to Heracles by King Eurystheus. Heracles had to capture the three-headed dog without using any weapons. After learning how the enter the Underworld, and assisted by some of the gods, he managed to go to the Underworld and find Hades to ask for his permission to take Cerberus to the surface. Hades agreed under the condition that Heracles must not use any weapons. Heracles eventually managed to overpower Cerberus and brought him to Eurystheus, successfully completing his twelve labours.
Cerberus is also called Kerberos.