Alternate Sara Lance (Caity Lotz) is a villainess from "Legends of To-Meow-Meow," episode 4.08 of Legends of Tomorrow (airdate December 10, 2018). She is a villainess from what would be the last alternate reality shown in the episode.
The episode had John Constantine, Charlie, and Zari Tomaz traveling through various timelines due to Constantine changing the past in the previous episode, "Hell No, Dolly!," by breaking up with his lover, Desmond, in order to save him. One reality had another villainous version of Sara Lance serving as the leader of the Sirens of Space-Time, a heel faction of vigilantes killing various magical beings. After various trips resulted in someone from the Legends (or connected to them) dying, their latest one resulted in no deaths, leaving Constantine and Charlie believing that things were normal.
After Constantine collapsed due to the various timelines being in his memory, he finally took Zuri's advice and went back to New Orleans to fix what he had done, leading to Charlie using her shapeshifting abilities to take the form of Sara's girlfriend, Ava Sharpe, and warn Sara about magical activity in New Orleans. Ray Palmer and Mick Rory were sent there with orders to shoot to kill, with Sara continuing to give the word after seeing three versions of Constantine in the area, believing they were shapeshifters. This alarmed Charlie to the point where she shifted from her Ava guise to the form of Amaya Jiwe, which she had been stuck in since episode 3.04, "Dancing Queen." Charlie asked a bewildered Sara her reasoning for killing magical beings, while also asking if she knew her actions were wrong. In response, Sara turned heel and snapped Charlie's neck.
However, Charlie fixed her neck and stood back up, prompting Sara (who gave the word to shoot the trio of Constantines, while one of them was with Desmond) to hold her at gunpoint and inform Charlie that she was never leaving the ship. When asked again by Charlie regarding her methods, the evil Sara stated that it was what kept them alive, before shooting Charlie. The shot failed to kill Charlie, further shocking Sara, and afterwards, Charlie realized that this evil and murderous version of Sara was a result of Constantine never joining the Legends, resulting in Charlie and the Legends never meeting and the Legends never learning that not all magical beings aren't a threat. The villainess referred to Charlie as a threat that needed to be taken out, leading to a fight between Sara and Charlie. However, when Desmond and Constantine kissed, the timeline was resorted to normal, erasing the villainous version of Sara.