Polydorus was the only son of Cadmus

Cadmus

Cadmus or Kadmos , in Greek mythology 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...

 and Harmonia

Harmonia (mythology)

In Greek mythology, Harmonia is the immortal goddess of harmony and concord. Her Roman counterpart is Concordia, and her Greek opposite is Eris, whose Roman counterpart is Discordia.-Origins:...

 and king of Thebes

Ancient Thebes (Boeotia)

See Thebes, Greece for the modern city built on the ancient ruins.Ancient Thebes was a Boeotian city-state , situated to the north of the Cithaeron range, which divides Boeotia from Attica, and on the southern edge of the Boeotian plain...

. His sisters were Semele

Semele

Semele , in Greek mythology, daughter of the Boeotian hero Cadmus and Harmonia, was the mortal mother of Dionysus by Zeus in one of his many origin myths. In another version of his mythic origin, he is the son of Persephone...

, Ino

Ino (Greek mythology)

In Greek mythology Ino was a mortal queen of Thebes, who after her death and transfiguration was worshiped as a goddess under her epithet Leucothea, the "white goddess." Alcman called her "Queen of the Sea" , which, if not hyperbole, would make her a doublet of Amphitrite.In her mortal self, Ino,...

, Agave

Agave (mythology)

In Greek mythology, Agave was the daughter of Cadmus, the king and founder of the city of Thebes, Greece, and of the goddess Harmonia. Her sisters were Autonoë, Ino and Semele, and her brother was Polydorus. She married Echion, one of the five Spartoi, and was the mother of Pentheus, a king of...

, and Autonoë

Autonoe

In Greek mythology, Autonoë was a daughter of Cadmus, founder of Thebes, Greece, and the goddess Harmonia. She was the wife of Aristaeus and mother of Actaeon and possibly Macris...

.

Upon his father's death, Pentheus

Pentheus

In Greek mythology, Pentheus was a king of Thebes, son of the strongest of the Spartes, Echion, and of Agave, daughter of Cadmus, the founder of Thebes, and the goddess Harmonia....

, the son of his sister Agave and Echion

Echion

In Greek mythology, the name Echion "son of the viper", cf. ἔχις echis "viper") referred to five different beings.*One of the Gigantes.*One of the surviving Spartoi, the "sown men" that sprang up from the dragon's teeth sown by Cadmus; "it was Echion who, for his great valor, was preferred by...

, one of the Spartoi, ruled Thebes. Pentheus, however, ruled only for a short time, before Dionysus

Dionysus

Dionysus was the god of the grape harvest, winemaking and wine, of ritual madness and ecstasy in Greek mythology. His name in Linear B tablets shows he was worshipped from c. 1500—1100 BC by Mycenean Greeks: other traces of Dionysian-type cult have been found in ancient Minoan Crete...

 caused his mother to tear him limb from limb. Polydorus succeeded him, marrying Nycteïs, the daughter of Nycteus

Nycteus

In Greek mythology, Nycteus was a king of Thebes. His rule began after the death of Polydorus, and ended when he was succeeded by his brother Lycus.-Genealogy:...

. When their son Labdacus

Labdacus

In Greek mythology, Labdacus was the only son of Polydorus and a king of Thebes. Labdacus was a grandson of Thebes' founder, Cadmus. His mother was Nycteis, daughter of Nycteus. Polydorus died while Labdacus was a young child, leaving Nycteus as his regent, although Lycus soon replaced him in that...

 was still young, Polydorus died of unknown causes, leaving Nycteus as his regent. In Pausanias

Pausanias (geographer)

Pausanias was a Greek traveler and geographer of the 2nd century AD, who lived in the times of Hadrian, Antoninus Pius and Marcus Aurelius. He is famous for his Description of Greece , a lengthy work that describes ancient Greece from firsthand observations, and is a crucial link between classical...

's history, Polydorus' rule began when his father abdicated, but this is the only source for such a timeline.