Kali was a Hindu Goddess and was one of many gods who met to try to work out a way to stop the Apocalypse. Along with Baldur, she appeared to be one of the most- if not the most- powerful and oldest of the gods. She had previously had a relationship with Gabriel, when she thought he was the Trickster demigod Loki. She bound Sam, Dean and Gabriel to her with their blood, making it impossible for them to leave.
In season 5, Kali helped organize the meeting at the Elysian Fields Motel. Dean first met her in the buffet area and attempted to hit on her, to which Kali simply responded, "No." She had Mercury gather blood from both Sam and Dean, so their souls are now forever bound to her by blood spell. Kali suggests - during the meeting - that they take violent actions towards the archangels. Kali obtained blood from Gabriel's vessel and bound him to her as well. She later killed him to show the remaining gods that with an Archangel sword, the gods can ultimately defeat Lucifer. Little did she know, Gabriel's sword was a fake, and Mercury would betray the rest of the gods and sell them out to Lucifer. Lucifer killed all the gods except Kali, who escaped with Sam and Dean while Gabriel confronted Lucifer.
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