Complex sentence: The earliest rationalized account (late 6th century BC) is that of Hecataeus of Miletus, in which Cerberus was a large poisonous snake, which lived on Tainaron, and was called the hound of Hades because anyone bitten by it died immediately.
Split into simpler sentences: The earliest rationalized account (late 6th century BC) is that of Hecataeus of Miletus. The serpent was called the "hound of Hades" because anyone bitten by it died immediately.