Dionysus is the child of Zeus and the mortal Semele. He is also known by the name Bacchus. He is the god of wine and viniculture. It seems that Dionysus is perhaps the combination of two gods. Dionysus is terribly unimportant to the early Greek writers. He is scarcely mentioned in both Hesiod and Homer, and seems to be very minor god.
Authors a few hundred years later treat Dionysus completely differently. Two new myths are seen about him, both of which are of a different nature than the earlier myths about a Hermes or Apollo. The story revolves around his death and rebirth. Zeus was madly in love with Semele. They had an affair, and Semele became pregnant. During the affair, he had promised her that he would do anything she wished. At the urging of the jealous Hera, Semele asked to see him in his true form. Although he did not want to, for it would kill the poor girl, Zeus had made a promise. Zeus did as he was asked, and the glory of the god killed Semele. Zeus, however, saved Semele's unborn child. He placed the child in his thigh until the time for him to be born. Then Hermes took the child down to earth and entrusted it to care of the nymphs of Nysa. It is from them that he was named. Here, he learned the art of viniculture. The story goes on to tell of Dionysus' struggles to be accepted as a god upon the world.
There is another story of Dionysus. Hera again tries to destroy Dionysus, this time the son of Zeus of Persephone. She enticed the child to the Titans, who then rip him to shreds. Athena, however, manages to rescue his heart. Zeus takes this heart, swallows it, and gives birth to a new Dionysus.
The death and rebirth, the struggle to introduce a new god, these aspects of his myth point towards Dionysus being the center of a mystery cult. This cult must have been introduced after the time of Hesiod and Homer. The introduction of this cult can be seen in a famous tragedy. The poet Euripides wrote a play, Bacchae, in the late fifth century, which details the dangers of the cult while describing the supposedly historical event involving the introduction of Bacchus.
Burkert, Walter. Ancient Mystery Cults. Harvard Press, 1987.