Cacus was a fire-breathing giant from Roman mythology and an enemy of Hercales - he was also the son of the Roman god Vulcan.
Cacus lived in a cave in the Palatine Hill in Italy, the future site of Rome. To the horror of nearby inhabitants, Cacus lived on human flesh and would nail the heads of victims to the doors of his cave. He was eventually overcome by Hercales.
According to Evander, Heracles stopped to pasture the cattle he had stolen from Geryon near Cacus' lair. As Heracles slept, the monster took a liking to the cattle and slyly stole eight of them - four bulls and four cows - by dragging them by their tails, so as to leave no trail. When Heracles awoke and made to leave, the remaining herd made plaintive noises towards the cave, and a single cow lowed in reply.
Angered, Heracles stormed towards the cave. A terrified Cacus blocked the entrance with a vast, immoveable boulder, forcing Heracles to tear at the top of the mountain to reach his adversary. Cacus attacked Heracles by spewing fire and smoke, while Heracles responded with tree branches and rocks the size of millstones. Eventually losing patience, Heracles leapt into the cave, aiming for the area where the smoke was heaviest. Heracles grabbed Cacus and strangled the monster, and was lauded throughout the land for his act. According to Virgil in Book VIII of his Aeneid, Heracles grasped Cacus so tightly that Cacus' eyes popped out and there was no blood left in his throat: "et angit inhaerens elisos oculos et siccum sanguine guttur."