Dionysus, also commonly known by his Roman name Bacchus, appears to be a god who has two distinct origins. On the one hand, Dionysus was the god of wine, agriculture, and fertility of nature, who is also the patron god of the Greek stage. On the other hand, Dionysus also represents the outstanding features of mystery religions, such as those practiced at Eleusis: ecstasy, personal delivery from the daily world through physical or spiritual intoxication, and initiation into secret rites. Scholars have long suspected that the god known as Dionysus is in fact a fusion of a local Greek nature god, and another more potent god imported rather late in Greek pre-history from Phrygia (the central area of modern day Turkey) or Thrace.
According to one myth, Dionysus is the son of the god Zeus and the mortal woman, Semele (daughter of Cadmus of Thebes). Semele is killed by Zeus' lightning bolts while Dionysus is still in her womb. Dionysus is rescued and undergoes a second birth from Zeus after developing in his thigh. Zeus then gives the infant to some nymphs to be raised. In another version, one with more explicit religious overtones, Dionysus, also referred to as Zagreus in this account, is the son of Zeus and Persephone, Queen of the Underworld. Hera gets the Titans to lure the infant with toys, and then they rip him to shreds eating everything but Zagreus' heart, which is saved by either Athena, Rhea, or Demeter. Zeus remakes his son from the heart and implants him in Semele who bears a new Dionysus Zagreus. Hence, as in the earlier account, Dionysus is called "twice born." The latter account formed a part of the Orphic religion's religious mythology.
It does seem clear that Dionysus, at least the Phrygian Dionysus, was a late arrival in the Greek world and in Greek mythology. He is hardly mentioned at all in the Homeric epics, and when he is it is with some hostility. A number of his stories are tales of how Dionysus moved into a city, was resisted, and then destroyed those who opposed him. The most famous account of this is that of Euripides in his play the Bacchae. He wrote this play while in the court of King Archelaus of Macedon, and nowhere do we see Dionysus more destructive and his worship more dangerous than in this play. Scholars have speculated not unreasonably that in Macedon Euripides discovered a more extreme form of the religion of Dionysus being practiced than the more civil, quiet forms in Athens.
Briefly, Dionysus returns to Thebes, his putative birthplace, where his cousin Pentheus is king. He has returned to punish the women of Thebes for denying that he was a god and born of a god. Pentheus is enraged at the worship of Dionysus and forbids it, but he cannot stop the women, including his mother Agave, or even the elder statesmen of the kingdom from swarming to the wilds to join the Maenads (a term given to women under the ecstatic spell of Dionysus) in worship. Dionysus lures Pentheus to the wilds where he is killed by the Maenads and then mutilated by Agave.
[text from Encyclopedia Mithica]