Pentheus was a king of Thebes, son of the Spartan Echion, and Agave (daughter of Cadmus, the founder of Thebes, and the goddess Harmonia).
Pentheus succeeded Cadmus as the king of Thebes and banned Dionysian worship in Thebes, ordering his soldiers to arrest anyone else found engaging in the rites. He saw the womens' (including his mother and aunts') divinely-caused insanity merely as drunken cavorting and an illicit attempt to escape the mores and legal codes regulating Theban society.
As a result of these actions Dionysus, wishing to punish Pentheus, lured Pentheus on mount Cithaeron where the Maenads (the female followers of Dionysus, including Perseus' mother Agave and his aunts) performed their bacchic rituals. Pentheus climbed up to the top of a tree for a better view of the Maenads but he was then spotted by the women who thought him to be a wild animal. Driven wild by this intrusion, the women tore the trapped Pentheus down and ripped his body apart, piece by piece (called "sparagmos" in Greek).
Pentheus' mother, Agave, still possessed by the Dionysian ecstacy, arrived back at the palace carrying the head of her son, believing it to be the head of a mountain lion which she had killed with her bare hands, ripping its head off, and she proudly displayed her son's severed head as a hunting trophy to her horrified father, Cadmus. As Dionysus' possession began to wear off, Agave slowly realized with horror what she had done.
Dionysus then appeared in his true form, and sent Agave and her sisters into exile, the family now all but destroyed. Still not satisfied, though, Dionysus chastised the family one more time for their impiety and, in a final act of revenge, turned Cadmus and his wife Harmonia into snakes.
As Dionysus and Tiresias later both point out, the name "Pentheus" means "Man of Sorrows"; even his name destined him for tragedy. The story of Dionysus punishing of sacriligeous Pentheus is most fully described in Euripides' play the Bacchae.