(The Sumerian form of Tammuz; Dumuzi-abzu)
"The Shepherd"
"Lord of the Sheepfolds"
"Quickener of the young in the mother womb of the deep"
Dumuzi was a Sumerian and Babylonian god of vegetation, pastures, fertility and the marshes. He later became associated with the underworld, where, as a companion of Nigizzida, he stood at the gates of the afterlife. Besides being a deity, he was also regarded as royal ancestor in several Mesopotamian city-states (in particular Uruk). He was a predecessor to Gilgamesh, a child of Dutter, and an eternal adversary of Enkimdu.
His wife was Inanna (the Sumerian form of Ishtar). Dumuzi was originally a mortal shepherd. Inanna married him and made him the king of Uruk. Because of their union the land prospered and was fertile. After a time, Inanna decided to visit her sister Ereshkigal, the goddess in the realm of death, so she could learn the secrets of the underworld. When she emerged from death, she found Dumuzi happy and celebrating and she was angered. To punish him for his unfeeling behavior she let demons carry him to the underworld.
Later, Inanna missed Dumuzi and she appealed to her sister to have him released. Ereshkigal allowed it, but only on the condition that Dumuzi’s sister Gestinanna would take his place for half a year. During late summer, fall and early winter, Dumuzi is held in the underworld apart from Inanna and no crops will grow. On new year's day he is released and Inanna rejoices, causing the crops to grow.
An alternate version of the myth has Inanna visiting the underworld to visit her sister. No mortal or god could leave once having stepped into the realm of death and she was trapped. The wise god Ea tricked Ereshkigal into returning Inanna to life, but someone had to take her place. Dumuzi went in her place for half of the year, and during this time no plants would grow.
Sources:
http://www.mesopotamia.co.uk/gods/explore/dumuzi.html
http://www.pantheon.org/articles/d/dumuzi.html
http://www.mystae.com/restricted/streams/scripts/tammuz.html
http://web.raex.com/~obsidian/MesoPan.html