In Greek mythology, Cadmus was a son of Agenor, himself a son of Poseidon. Agenor also had two other sons, Phoenix and Cilix, and a daughter, Europa. Horny old Zeus had the hots for Europa and disguised himself as a beautiful white bull; he met Europa at the seashore and, feigning tameness, coaxed her onto his back, whereupon he swam away across the sea to Crete. Europa bore Zeus three sons, and received from him three gifts: a bronze man who guarded her; a dog which could catch any quarry, and a javelin which never missed its mark. Later she married the king of Crete, after Zeus' eye wandered, I guess. But I digress...
Agenor was pretty pissed off that his daughter had been abducted - for that's what it was, Zeus or no - and sent his three sons off to find her, prohibiting them from returning until she was found. Well, it was a hopeless quest, and they were basically exiles from their home; Phoenix founded Phoenicia, Cilix Cilicia, and Cadmus Thebes. (Why not Cadmia? I don't know.) He didn't just found it, though; he consulted the oracle at Delphi and received a typically gnomic utterance, the gist of which was that he should follow a cow that he would soon see, and where it lay down, he should found a city. So he and his men followed a cow until it lay down, and when it did, he ordered his men to bring water so that he could offer a sacrifice to Athena, as I suppose was the custom, but unfortunately they encountered a dragon or serpent which was sacred to Ares, and were all killed. Cadmus went looking for his comrades and found them slain; he engaged in a fearful battle of his own, eventually slaying the serpent. He then heard a mysterious prophesy that he himself would one day become a serpent.
No doubt he was disturbed by this news, but he had other matters to attend to first, like that city he was supposed to found, but with no manpower now, all his men being dead. Athena, perhaps feeling a little guilty over having been the cause of all this slaughter, advised Cadmus to sow the serpent's teeth in the earth, which he did, and armed men sprang up from the planted teeth. Unfortunately they were a quarrelsome lot, and immediately fell to fighting; in the end only five remained, and these became the ancestors of the aristocratic families of Thebes. Poor Cadmus, though, had to spend eight years in servitude to Ares because he'd killed his precious serpent. Vengeful lot, those Greek gods.
After his indenture was up, he got to be ruler of Thebes and married Harmonia, daughter of Ares and Aphrodite; they had four daughters and a son. When Cadmus and Harmonia grew old they moved to Illyria, where they were transformed into serpents, just as the voice had foretold so long before.
Besides his adventures with serpents and gods, Cadmus introduced into Greece the sixteen simple Cadmean letters of the Greek alphabet.
Thanks to the Encyclopedia Mythica (www.pantheon.org/mythica.html)