Atropos is the main antagonist of the Supernatural epispde "My Heart Will Go On". She is one of the Fates - one of those in charge of the inevitability of death along with her sisters, who implement how mortals and people would die.
Prior to the Apocalypse, the sisters followed God's design perfectly, with Atropos stating that God gave them their duty personally. When the Apocalypse approached, the sisters had arranged everything as it was supposed to be for the final battle. Things changed when Castiel, Sam and Dean Winchester averted the Apocalypse and threw everything into chaos. Atropos explained that her sisters were willing to continue as things were and went back to work, but Atropos had always been a perfectionist and went to Heaven expecting instructions. What she found there was the chaos of the Civil War and, with everyone ignoring her, she was forced back to work to which she stated she didn't make a fuss about. This was not to last, when time was altered and history was rewritten.
In a desperate gambit, Castiel ordered Balthazar to go back in time to save the Titanic so that he may have more souls to aid him in battle against Raphael. This created a new timeline where nearly 50,000 new souls were created and both Ellen and Jo Harvelle were still alive. For Atropos, this was the last straw. While she had been willing to let alterations to a future event slide, altering a past event was unacceptable. Atropos began hunting down the descendants of the Titanic and killing them in order to clean up the "mess" Castiel and Balthazar had created.
Her actions eventually drew the attention of the brothers, who narrowly avoided being killed by her due to her ability to stop time and set up disasters. Castiel managed to save the brothers and, while lying about his involvement in the events, told the brothers that they are very high on Atropos' hit list since they stopped the Apocalypse and made her obsolete. He told them that once Atropos had a target, she wouldn't stop until that person was dead (so the brothers would have to kill her first). The group set up a trap where Atropos prepared to drop kitchen appliances on the brothers only for Castiel to arrive and confront her. Castiel tried to talk her down but she angrily berated him for "Tempting Fate" by messing with the Titanic and that she was going to fix the mess no matter what. When Castiel told her that he would stop her, she reminded him that her sisters have been compliant but her death would galvanize them into action. While Atropos was quite powerful, her sisters "are bigger in every sense of the word" and would kill Dean and Sam faster than Castiel could blink before finishing Atropos' work.