Astaroth was a powerful demon who worked to condemn human souls to Hell by turning them into witches. She turned Renee Van Allen, Elizabeth Higgins, and Amanda Burns into witches, which resulted in several deaths, including the entire coven in question. The Winchesters' investigation led them to Astaroth and they ended up killing her. She was played by Marisa Ramirez.
Astaroth was hundreds of years old. In the fourteenth century, she came into contact with a human whom she taught witchcraft to in exchange for the human's soul, which went to Hell and became another demon, named Ruby.
Astaroth herself later ended up in Hell. She managed to escape when a Devil's Gate was opened in "All Hell Breaks Loose - Part 2"; when talking about the escape later with Ruby, Astaroth described escaping Hell as "a bitch of a fight" and was impressed that her student had made it.
Astaroth was a devout follower of Lilith. In an attempt to please Lilith, Astaroth and her "friends" sought to hunt down and kill Sam Winchester in order to eliminate Lilith's "competition" for leadership of Hell.
Astaroth secretly possessed Tammi Fenton in order to "break the ice" with Tammi's friends Amanda Burns, Elizabeth Higgins, and Renee Van Allen. She then brought the Book of Shadows into the women's book club, and tricked them into unwittingly selling her their souls in exchange for her improving their lives.
Amanda used witchcraft to kill her lover Paul's wife, and then tried to kill Paul himself, bringing Sam and Dean Winchester to investigate. Astaroth planted a hex-bag that killed Amanda in a way that appeared to be a suicide, and later cast a spell to kill Dean, although Ruby intervened to break the spell and save Dean.
When Sam deduced that Tammi was possessed, Astaroth briefly kept up her charade before dropping it and taunting him by claiming that his brother was already dead. Sam tried to shoot her with the Colt, but she telekinetically slowed and then stopped the bullet completely, making it fall to the floor without hitting her. She incapacitated him, killed an angry Renee out of annoyance, and taunted a horrified Elizabeth about having sold her soul without realizing it.
Astaroth started trying to kill Sam by crushing him to death. When Dean and Ruby arrived, Astaroth incapacitated Dean as well, but allowed Ruby to explain why she was there; Ruby claimed to have lured the Winchesters to Astaroth as a gift and that she wanted to serve Astaroth again. Astaroth appeared to consider Ruby's words for a moment, but then Ruby tried to kill her, leading Astaroth to brutally beat Ruby. She began to exorcise the younger demon, only to be caught off-guard by Elizabeth's spell, causing her to release Sam and Dean and kill Elizabeth. Her brief distraction allowed Dean to retrieve the demon-killing knife, grab her from behind and stab her several times, killing her