According to Diodorus Siculus, Lamia was born the beautiful daughter of King Belus of Egypt, making her the granddaughter of Poseidon and Lybie. Upon her father's death she became queen of one of his territories, Libya. However, while visiting Delphi, Pausanias remarks Lamia was the daughter of Poseidon. He also states Lamia and Zeus were the parents of Herophile, a noted sibyl.
Diodorus goes on to relate Lamia had an affair with Zeus and bore him children. When Hera, Zeus' wife, discovered the affair, she became enraged and killed the children. Driven insane with grief, Lamia began devouring other children, and, according to Diodorus, her face became hideously distorted from her grisly deeds. Zeus then gave her the ability to remove her eyes. In Diodorus the purpose of this is unclear, but other versions state this came with the gift of prophecy. Zeus did this to appease Lamia in her grief over the loss of her children.
Later stories state Lamia was cursed with the inability to close her eyes so that she would always obsess over the image of her dead children. Some accounts (such as that of Horace, below) say Hera forced Lamia to devour her own children. Myths variously describe Lamia's monstrous (occasionally serpentine) appearance as a result of either Hera's wrath, the pain of grief, the madness that drove her to murder, or - in some rare versions - a natural result of being Hecate's daughter.