Kali

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. In the episode, Hammer of the Gods, she bound Sam, Dean and Gabriel to her with their blood, making it impossible for them to leave.
She revealed that she had discovered that Gabriel was an Archangel, and she used what she thought was an Angel Killing Sword to kill him. However the sword was a fake, and he survived, later saving her from Lucifer after he had killed the other gods. She escaped with Sam and Dean and her current wherabouts are unknown.