According to the Toreador loremaster Katherine of Montpelier, Tammuz was a mortal lover of the Toreador Antediluvian (who was known as Ishtar by the time she fled the Second City in order to reside in Sumer). The people worshiped the antediluvians beauty and made her their goddess.
She soon Embraced her lover Tammuz, and when Ishtar's Kindred siblings discovered that she was in Sumerian territory, they became outraged. It was Ennoia, "The Beast" Gangrel, who set out to kill the Toreador Antediluvian. As Ennoia arrived in Sumer, she Embraced Ereshkigal to help her in the attack.
When they came to confront Ishtar, Tammuz interceded. He sacrificed himself facing Final Death so that Ishtar could flee. She then sought aid from the king of Sumeria, Gilgamesh. However, he had already been turned by Ennoia.
In Ishtar's despair, she called upon The Bull of Heaven who helped her by means of escaping to Crete.
Clanbook: Toreador Revised, p. 15, 16