Cadmus or Kadmos (Greek: Κάδμος), in Greek, Roman and Phoenician mythologies, was a Phoenician prince, the son of king Agenor and queen Telephassa of Tyre and the brother of Phoenix, Cilix and Europa. He was originally sent by his royal parents to seek out and escort his sister Europa back to Tyre after she was abducted from the shores of Phoenicia by Zeus. Cadmus founded the Greek city of Thebes, the acropolis of which was originally named Cadmeia in his honor.
Cadmus was credited by the ancient Greeks (Herodotus is an example) with introducing the original Alphabet or Phoenician alphabet -- phoinikeia grammata, "Phoenician letters" -- to the Greeks, who adapted it to form their Greek alphabet. Herodotus estimates that Cadmus lived sixteen hundred years before his time, or around 2000 BC.
Though later Greeks like Herodotus dated Cadmus's role in the founding myth of Thebes to well before the Trojan War (or, in modern terms, during the Aegean Bronze Age), this chronology conflicts with most of what is now known or thought to be known about the origins and spread of both the Phoenician and Greek alphabets. While a Phoenician origin for the Greek alphabet is certain, the earliest Greek inscriptions match Phoenician letter forms from the late 9th or 8th centuries BC -- and, in any case, the Phoenician alphabet properly speaking wasn't developed until around 1050 BC (or after the Bronze Age collapse). The Homeric picture of the Mycenaean age betrays extremely little awareness of writing, possibly reflecting the loss during the Dark Age of the earlier Linear B script. Indeed the only Homeric reference to writing  was in the phrase "γράμματα λυγρά", grámmata lygrá, literally "ominous etchings", when referring to the Bellerophontic letter. (According to Walter Burkert in The Orientalizing Revolution, literacy explodes within a few decades after 750 BC: "The earliest Greek letters recognized to date originate in Naxos, Ischia, Athens, and Euboea, and appear around or a little before 750".) Linear B tablets have been found in abundance at Thebes, which might lead one to speculate that the legend of Cadmus as bringer of the alphabet could reflect earlier traditions about the origins of Linear B writing in Greece (as Fred Ahl speculated in 1967). But such a suggestion, however attractive, is by no means a certain conclusion in light of currently available evidence. The connection between the name of Cadmus and the historical origins of either the Linear B script or the later Phoenician alphabet, if any, remains elusive. However, in modern day Lebanon, Cadmus is still revered and celebrated as the 'carrier of the letter' to the world.
According to Greek myth, Cadmus's descendants ruled at Thebes on and off for several generations, including the time of the Trojan War.
Founder of Thebes
Cadmus came in the course of his wanderings to Delphi, where he consulted the oracle. He was ordered to give up his quest and follow a special cow, with a half moon on her flank, which would meet him, and to build a town on the spot where she should lie down exhausted.
The cow was given to Cadmus by Pelagon, King of Phocis, and it guided him to Boeotia, where he founded the city of Thebes. Robert Graves (The Greek Myths) suggested that the cow was actually turned loose within a moderately confined space, and that where she lay down, a temple to the moon-goddess (Selene) was erected: "A cow's strategic and commercial sensibilities are not well developed," Graves remarked.
Intending to sacrifice the cow to Athena, Cadmus sent some of his companions to the nearby Castalian Spring, for water. They were slain by the spring's guardian water-dragon (compare the Lernaean Hydra), which was in turn destroyed by Cadmus, the duty of a culture hero of the new order.
By the instructions of Athena, he sowed the dragon's teeth in the ground, from which there sprang a race of fierce armed men, called the Spartoí ("sown"). By throwing a stone among them, Cadmus caused them to fall upon one another until only five survived, who assisted him to build the Cadmeia or citadel of Thebes, and became the founders of the noblest families of that city.
The dragon had been sacred to Ares, so the god made Cadmus do penance for eight years by serving him. According to Theban tellings, it was at the expiration of this period that the gods gave him Harmonia ("harmony", literally "well put together", or "well assembled") as wife. At Thebes, Cadmus and Harmonia began a dynasty with a son Polydorus, and four daughters, Agave, Autonoë, Ino and Semele.
At the wedding, whether celebrated at Samothrace or at Thebes, all the gods were present; Harmonia received as bridal gifts a peplos worked by Athena and a necklace made by Hephaestus. This necklace, commonly referred to as the Necklace of Harmonia, brought misfortune to all who possessed it. Notwithstanding the divinely ordained nature of his marriage and his kingdom, Cadmus lived to regret both: his family was overtaken by grievous misfortunes, and his city by civil unrest. Cadmus finally abdicated in favor of his grandson Pentheus, and went with Harmonia to Illyria, to fight on the side of the Encheleans later as king he founded the city of Lychnidos and Bouthoe.
Nevertheless, Cadmus was deeply troubled by the ill-fortune which clung to him as a result of his having killed the sacred dragon, and one day he remarked that if the gods were so enamoured of the life of a serpent, he might as well wish that life for himself. Immediately he began to grow scales and change in form. Harmonia, seeing the transformation, thereupon begged the gods to share her husband's fate, which they granted (Hyginus).
In another telling of the story, the bodies of Cadmus and his wife were changed after their deaths; the serpents watched their tomb while their souls were translated to the fields. In Euripides' The Bacchae, Cadmus is given a prophecy by Dionysus whereby both he and his wife will be turned into snakes for a period before eventually being brought to live among the blest.