(illustration by William Blake of Cerberus, the three-headed dog who guards the gate of Tartarus)

Not all demons are humanoid in appearance. Cerberus is the Greek demon who stands faithful guard at the adamantine gate to the underworld to prevent the souls of the dead from escaping back up to the sun-lit surface of the Earth, and also to prevent the living from freely entering and leaving Tartarus. It has three heads, a serpent tail and a mane of serpents. In its earliest form Cerberus had fifty heads, but these were later reduced to three.