Ishtar is the Assyrian and Babylonian version of the fertility goddess, sharing local goddess space with the Sumerian Innanna, and Semitic goddess Astarte (Ashtoret in Hebrew). Though each goddess had local flair, they largely reflected a similar view of the goddess and even shared many specific myths of her doings. In addition to her role as a fertility goddess, she was also the war goddess.
The Epic of Gilgamesh describes her as a petulant and spoiled brat said to have killed or transformed into beasts her mortal lovers. Once Gilgamesh refuses to be Ishtar's next sex toy, she sends against him the Bull of Heaven but not before threatening other gods not very supportive of Ishtar's actions to open the gates of Hell/the Underworld and having the dead outnumbering the living (yes, sounds very familiar). Remember that the Underworld hath no fury like a goddess scorned[notes 1].
Innana, was both the most powerful woman in Sumerian religion and also unwed. She was at times considered more powerful than the most powerful god; Enki. Her rituals consisted of the cross-dressing of cult personnel, rituals "imbued with pain and ecstasy, bringing about initiation and journeys of altered consciousness; punishment, moaning, ecstasy, lament and song, and participants exhausting themselves with weeping and grief." 
Centuries after she had faded out in relative obscurity, she was mixed in with the Easter festivity by Alexander Hislop on his book The Two Babylons because the two names sound similar, something rejected by modern scholars as a good pile of manure, and has also ended up being present in Neopaganism even if that's a theme park version that has her more masculine elements (warfare, etc) removed.
- According to the myth, she attempts the same stunt on her other descent to the Underworld in order to be able to enter there. While it works, she is considerably less lucky once inside. See the other Wiki.