Tammuz, also known as Dumuzid, was the Mesopotamian pastoral god of shepherds and fertility. He was the son of Sirtur and an unknown father (sometimes speculated to be Ea), brother of Geshtinanna, and consort of Inanna.
Tammuz was the consort of the great goddess Ishtar, and their relationship was the subject of many ancient hymns. When Ishtar perished and descended into the underworld, Tammuz remained above enjoying the great luxuries of her divine palace. Ishtar would eventually rise from the underworld, and return to the world of the living and was shocked to find out that her consort was not mourning and weeping for her. This led to Ishtar deciding that Tammuz would take her place in the underworld.
Tammuz fled, and his sister Gestinanna stepped into the picture, negotiating on her brothers behalf with both Ishtar and the gods of the underworld. It was agreed that he would spend half of the year in the realm of the dead and half in the world of the living, representing the yearly cycle of the weather. A fertility cult formed around Tammuz, that was even practiced among the ancient Hebrews, as mentioned in Ezekiel 8:14 of the Bible.
Tammuz would later be seen as a demonic being in Christian theology, such as in John Milton's epic poem Paradise Lost (alongside Moloch, Chemosh, Astarte, Baal, Dagon, Rimmon, Osiris, Isis, Saturn, Jupiter and Vulcan) and according to the Dictionnaire Infernal, he was the demonic inventor of artillery and Hell's ambassador to Spain.