Trickster's Brigade

The Trickster's Brigade were a group of beings led by the Trickster.

History
Like their leader, the Brigade sought to change the course of history by altering details in individuals' lives. According to the Tenth Doctor, most of these changes were "tiny" ones the universe could "compensate" for. However, if the Brigade's victim had a particularly great influence on the universe, the change would create an entire parallel world around that person. (TV: Turn Left)

In 1927, the Brigade smuggled a brain parasite to Earth, intending to infect the future US President Franklin D. Roosevelt. The plan was that he would be driven mad and the United States and its allies would be unable to stop Nazi Germany in the future, ensuring their victory in the Second World War. This plan was foiled by Captain Jack Harkness and Angelo Colasanto. (TV: Immortal Sins)

The Trickster altered timelines by swapping Sarah Jane Smith for her school-friend Andrea Yates: during a cliff-side accident on 13 July 1964, the Trickster caused Sarah Jane to die instead of Andrea. The timeline was restored when Andrea Yates ended the agreement she had made with the Trickster during the accident. (TV: Whatever Happened to Sarah Jane?)

Time Beetles were identified by the Doctor as "one of the Trickster's Brigade". (TV: Turn Left)

One of the Brigade's attempts to radically change the universe created a world where Donna Noble never met the Doctor and he died. The Doctor's companions stopped some of the threats to Earth, but all died (or in Jack Harkness's case, were trapped on another world) doing so until only the alternate Donna Noble and Rose Tyler from the correct universe remained. Rose convinced the alternate Donna Noble to travel back in time with an improvised time machine and reverse the Brigade's effects by sacrificing herself to force her past self to change her mind. (TV: Turn Left)

Behind the scenes
The Trickster's Brigade are the only enemies to be faced in Doctor Who, Torchwood and The Sarah Jane Adventures.