In Virgil's account, Cacus was born of divine fire. His father was Vulcan, the Roman god associated with fire and metalworking and identified with the Greek Hephaestus. The son turned that power towards destruction, belching smoke and flame from a cavern on the Aventine. Sunlight never reached its depths, and the traces of his victims made the entrance a warning to the surrounding people. Virgil describes a monstrous being, partly human and partly beast, whose strength was matched by his brutality. This is the giant remembered in the best-known literary telling, not a description shared by every Roman account. The cave and its threatening occupant belong to the imagined landscape before the foundation of Rome.

The trouble began when Hercules came to Italy with the cattle of Geryon. The hero had defeated the formidable guardian of the herd in the far west as part of his labours and was driving the animals on his return journey. Near the Tiber, on ground that would later belong to Rome, the cattle rested and grazed. Cacus saw an opportunity. The beasts were valuable, and the passing hero seemed to offer a prize that could be stolen without an open contest of strength. The encounter links the long journey from Geryon's distant land with a local Roman story. It is an episode on the return from the cattle labour, not a separate thirteenth task imposed upon Hercules.

Virgil gives the theft a precise shape: Cacus took four bulls and four heifers and dragged them backwards by their tails into his cave. Anyone following the hoofprints would see tracks leading out rather than in. The trick almost worked. As Hercules prepared to move the remaining herd onwards, the animals lowed, and one of the hidden cattle answered from the cavern. Sound revealed what the tracks concealed. The hero understood the deception and rushed towards the lair. Cacus fled inside and sealed the entrance with a huge stone suspended by chains, trusting the rock and his inherited fire to keep the intruder out. His theft had become a siege.

Hercules could not force the entrance, despite repeated efforts. He searched the hill and found a projecting mass of rock above the cavern. Straining against it, he tore it loose and exposed the hidden chamber to daylight. Cacus was trapped beneath the opening, but he filled the space with smoke and fire until the hero could scarcely see his enemy. Hercules plunged into the darkness, seized him and crushed his throat. Virgil makes the struggle violent and physical: the smoke that had protected the robber could not save him once the stronger opponent reached him. The stolen cattle were recovered, and the people came to look upon the body of the being who had terrorised them. The cavern had ceased to be a hiding place.

For Evander, who tells the story in the Aeneid, the victory explains the community's gratitude and its continuing rites for Hercules. The Ara Maxima, the Greatest Altar associated with the Forum Boarium, gives the tale a place in Roman sacred memory. The Potitii and Pinarii appear in the traditions surrounding the worship and its ceremonial responsibilities. These accounts connect an established cult with a heroic foundation story; they do not provide historical proof that a fire-breathing giant occupied the hill. The significance lies in the connection between place and narrative. A site known for worship and sacrifice could be remembered as the ground where a dangerous stranger had been defeated and a protecting hero honoured.

The surviving versions do not tell an identical story. Livy presents Cacus as a strong local shepherd rather than Virgil's fire-breathing monster. His Hercules is asleep after food and wine when the cattle are stolen, discovers the theft through their lowing and kills the resisting thief with a blow of his club. Evander then recognises the hero and connects him with the promised divine honours. Virgil instead gives the encounter the scale of a struggle against a monstrous enemy and places it within Evander's welcome to Aeneas. Neither version needs to erase the other. Together they show how the same cattle theft could be shaped as local legend or epic spectacle, while preserving the association between Hercules and the future site of Rome.