Short answers, pulled from the story.
Pentheus was the king of Thebes and the grandson of the city's founder Cadmus. He is known primarily from Euripides' tragic play The Bacchae, which dramatizes his refusal to permit the worship of Dionysus and his violent death as a result.
The name Pentheus derives from the Greek word penthos, meaning sorrow or grief, particularly the grief caused by the death of a loved one. Both Dionysus and the seer Tiresias drew attention to this meaning within the story itself.
Pentheus banned the worship of Dionysus after inheriting the throne of Thebes from his grandfather Cadmus and forbade the women of Cadmeia from participating in the god's rites. He did this despite the fact that Dionysus was his own cousin, the son of his aunt Semele.
Pentheus was lured by Dionysus to spy on the Bacchic rites while disguised as a woman, and climbed a tree to watch. The daughters of Cadmus, in a Bacchic frenzy, mistook him for a wild animal and tore him apart in a ritual act called the sparagmos. His mother Agave was the first to attack him, tearing off his arm and then his head.
In Ovid's Metamorphoses, Pentheus's guards arrest a sailor named Acoetes of Maeonia rather than Dionysus. Acoetes tells how Bacchus turned his crew into dolphins, and when the guards try to shackle him the chains fall off. Pentheus then charges into the Bacchanalia himself and is killed by the worshippers, who mistake him for a boar.
After Marcus Licinius Crassus was killed at the Battle of Carrhae in 53 BC, Plutarch records that his severed head was sent to the Parthian emperor Orodes II and used as a stage prop in a production of The Bacchae, standing in for the head of Pentheus. A classical play in the Meitei language also retells the myth, casting Pentheus not as an individual king but as a small community threatened by the force of developed nations.