Future Android 18

Future Android 18 is a much weaker, but more evil, alternate counterpart of Android 18 in the History Of Trunks and in the episode, "Ghosts of Tommorrow". She serves as one of the two main antagonists (the other one being her brother) of the Dragon Ball Z special The History of Trunks.

Personality
In contrast to her main timeline self, Future Android 18 and her brother are extremely sociopathic and cruel, taking joy in killing and destruction. According to them, their personalities are the direct result of Future Dr. Gero's attempts at programming them to hate humans as part of his bid to conquer the world. Despite her brutal and cruel nature, Future 18 has shown instances of self-restraint, such as scolding her brother for murdering a clothing store salesperson despite intending to let him live.

Story
Six months after the Goku's tragic death in Age 766 in an alternate timeline, Future Android 17 and Future Android 18 are released by Future Dr. Gero to kill Goku as part of his plot for revenge. According to Super Perfect Cell's flashback, Future Android 17 and Future Android 18 are willingly wandering through Dr. Gero's Laboratory as Future Dr. Gero is working on his "ultimate android creation". Future Android 18 knocks over some glass, Gero attempts to use his remote but Future Android 17 decapitates Gero. As Gero's decapitated head says that his "ultimate android creation" will absorb them, Future Android 18 blasts the head, as well as destroyed Cell's chamber in an attempt to destroy Cell himself. Then, the two androids head off to Amenbo Island, and they begin their reign of destruction.

Soon after, coming to the defense of the inhabitants, the Z Fighters challenge the ruthless killing machines. Future Android 18 kills Future Piccolo with a powerful kick to the abdomen, Future Tien Shinhan with a punch to his gut followed by a High-Pressure Energy Wave, and Future Yajirobe by breaking his katana before killing him with a High-Pressure Energy Wave too (this is a coincidence given the fact that Android 18 breaks Future Trunks' sword as well). The androids killed every single Z Fighter, including Vegeta and Piccolo, except for Gohan. The Z Fighters are unable to be wished back as the Dragon Balls vanish forever, due to Piccolo's death which also kills Kami. All in some thrill-seeking fun...

Trivia

 * Despite both she and her brother being the main antagonists of The History of Trunks, 18 seems to be the secondary antagonist of the movie and her brother being the bigger bad main antagonist due to 17 enjoying killing innocent people more than his sister, and 17 doesn't even spare one human, which 18 did do one until 17 killed the victim, and 18 even said, "17 no I spared him, you made me look bad".