The Trickster

The Trickster was an immortal extra-dimensional alien, formerly part of an extra-dimensional group of beings known as the Pantheon of Discord, who were banished from the known universe. The Trickster, alone, frequently manifested on Earth, making bargains to alter history in attempts to draw power from the chaos he created and thus enter the world in his full power.

Biography
Not much was known about the Trickster's origin, other than it was affiliated with the group known as the Pantheon of Discord. All that was recorded was his attempts to influence history. (TV: The Wedding of Sarah Jane Smith)

At some point, the Trickster learned that the Graske Krislok was destined to die, and set about trying to prevent this. Projecting an image of himself into Krislok's spacecraft, mere moments before it was about to burn up, the Trickster offered to save Krislok from death if he gave him his consent. Krislok agreed, and the Trickster rescued him. In exchange for saving him, the Trickster forced Krislok to become his slave. (TV: The Temptation of Sarah Jane Smith)

First attack
Later on, to satisfy his need for chaos, the Trickster found an event in 2008, which would have caused destruction on Earth and wiped out humanity, who would otherwise become a major species in the universe. This event was a meteorite crashing into the Earth, undetected by military radar. (TV: Whatever Happened to Sarah Jane?) There were many other times when life on Earth would have been threatened by alien life, (TV: Invasion of the Bane, Revenge of the Slitheen, Eye of the Gorgon, et al) but the meteorite crash was one of the few with no purpose, the result of pure chance. This would ultimately give the Trickster the opportunity to consume the resulting chaos, and manifest himself in reality in full power. (TV: Whatever Happened to Sarah Jane?)

In the original timeline, the impact would have been stopped by Sarah Jane Smith and Mr Smith. The Trickster removed Sarah Jane Smith from Earth's timeline, influencing events so she died in the place of Andrea Yates (who should have died in the original timeline) in 1964.

The Trickster also forced the aliens that Sarah Jane would have faced to leave the Earth alone. As a result, Luke Smith, Sarah Jane's adopted son, was never born because the Bane never invaded Earth. However, all did not go to plan. When removing Sarah Jane from history, Maria Jackson held an alien puzzle box, protecting her from the effects and allowing her to remember the original timeline. She reminded Andrea Yates of the day Sarah Jane died, causing Andrea trauma. The Trickster sent his Graske servant Krislok to remove Maria from history. However her father found and held the cube with Andrea's consent, allowing him to remember the events. In turn, he reminded Andrea of Maria, again causing her distress. The Trickster sent Krislok after Alan, but he foiled the plan and trapped the Graske.

Alan found a device on Krislok which summoned Maria back to existence from Limbo, and they went to Sarah Jane's house (in this time line, Andrea's house) where Sarah Jane appeared in a mirror. She explained that while Andrea was alive, Sarah Jane couldn't be. Andrea, realising that she had to die (either by falling off a pier in 1964 or in the soon-to-be meteorite crash), repudiated her deal with the Trickster. The timeline returned to normal, Sarah Jane and Mr Smith deflected the incoming meteorite and the Trickster was defeated. Through Sarah Jane's memories, the Trickster also learned of the existence of the Doctor and planned on using Sarah Jane to remove him from time, which would have resulted in an untold amount of chaos. (TV: Whatever Happened to Sarah Jane?)

Revenge
The Trickster decided to get revenge on Sarah Jane. He opened a time portal to 1951, where she went to meet her parents, whom she did not remember. Sarah Jane and Luke deactivated her father's car, saving her parents from death. By preventing a fixed event in history, the Trickster was able to physically manifest on Earth in nearby sacred ground, creating an alternate time line in which the Earth was drained of its life by the Trickster, and his Graske, Krislok, had enslaved mankind, forcing them to mine for minerals to give the Trickster power to escape into space and enslave other planets. However, Rani Chandra travelled back and warned Sarah Jane of the Trickster's plan, and her parents sacrificed themselves to correct the time line. (TV: The Temptation of Sarah Jane Smith)

An uninvited wedding guest
When Peter Dalton fell down the stairs at home and was about to die, the Trickster — clad in white as opposed to his usual black — appeared to him and asked for his agreement. If he agreed, he would survive but would have to get Sarah Jane to marry him, which he did. However, the wedding was disrupted by the sudden arrival of the Tenth Doctor, who had learned of the Trickster's plan and demanded the wedding be stopped. At this point, the Trickster trapped them in two time loops, with Peter and Sarah Jane in one and the Doctor, Luke, Clyde, Rani and K9 in another. If Sarah Jane did not agree to the wedding, they would all be trapped forever. If married, Sarah Jane would stop fighting aliens, and chaos would ensue.

The Doctor's TARDIS tried to get into the time loop, and the Doctor was able to get inside, but the TARDIS was pulled out of the loop before the others could be saved. When Clyde absorbed artron energy from the leaving TARDIS, he attacked the Trickster, weakening them both. He and Clyde were sent back in the reception room as the Doctor arrived. When Peter learned the full story and the consequences of his action, he repudiated his deal and threw his ring at the Trickster, banishing him. (TV: The Wedding of Sarah Jane Smith)

Personality
The Trickster was an enigmatic and sinister creature that was perpetually hungry for chaos, which he reveals to be what provides him with energy. He was highly intelligent and manipulative, capable of influencing anyone with words. He was arrogant, thinking himself as superior to all beings and taking all other life forms for granted. This proved to be his weakness every time he faced Sarah Jane Smith: He was incapable of understanding, or even considering, the full extent of human emotion- he only preyed on the human instinct to live, which is shown when he always meets his victims and tries to manipulate them on the point of death.

Abilities
The Trickster was an incredibly powerful creature. He fed on chaos and acted to increase random chaos. He had less interest in the chaos created by intelligent beings, preferring the chaos of natural disasters. (TV: Whatever Happened to Sarah Jane?) The Trickster was also extremely manipulative: He had power over people, but only with their consent. He tended to go after the dying, giving them life in return for servitude. (TV: Whatever Happened to Sarah Jane?, The Temptation of Sarah Jane Smith, The Wedding of Sarah Jane Smith)

Appearance
The Trickster had a humanoid form, but with sharp teeth and an otherwise featureless face. The Trickster normally wore a long black hooded robe and gloves, covering his entire body apart from his face. (TV: Whatever Happened to Sarah Jane?, The Temptation of Sarah Jane Smith) Whether this was his true form or an attempt to cause fear is unknown. (WC: The Alien Files) When he appeared to Peter Dalton under the pretence that he was a benevolent angel there to help, he wore the same outfit, but the colour was entirely changed to white; resuming his more customary black only while apart from Peter and Sarah Jane. (TV: The Wedding of Sarah Jane Smith)

Other influences
There were other members of the Trickster's Brigade. One was a beetle-like creature which created a parallel world around Donna Noble in which the Tenth Doctor died. This resulted in much chaos due to alien attacks and incursions on Earth not being thwarted. (TV: Turn Left) Others organised a plot involving a Brainspawn driving future American President Franklin Delano Roosevelt insane, causing him to drop America out of World War II resulting in Nazi Germany winning the war. This plan was foiled by Jack Harkness and Angelo Colasanto. (TV: Immortal Sins)

Other information
When cyborg hacker Psi was attempting to protect Clara Oswald from the Teller, he ensured that it would focus on his guilt by uploading the memories of some of the worst criminals in history, one of them being the Trickster. (TV: Time Heist)

Behind the scenes

 * The Trickster is the villain of a Doctor Who: Legacy storyline, named "The Trickster Event".
 * Initially the character's name was only given to the being by Alan Jackson as a description of him and was referred to by that title in the end credits. In The Temptation of Sarah Jane Smith, he called himself "the Trickster". He was specifically named "the Trickster" by the Tenth Doctor in TV: Turn Left and The Wedding of Sarah Jane Smith. Jack Harkness also referred to him as the Trickster in Immortal Sins.
 * It's implied twice that the Trickster was one of the Eternals: He calls Sarah Jane Smith an "Ephemeral", an expression the Eternals use to describe mortals, and the Doctor refers to the Trickster as being "an eternal exile"; it is unknown whether or not he was saying the Trickster was in fact, an exile of the Eternals or just somebody who's exiled for Eternity.
 * A running gag occurs when the person realised the Trickster tricked them, they'll say "You tricked me!"
 * The Trickster would have returned in the series 5 finale The Battle for Bannerman Road. It would have been revealed he used a dimensional schism above the fleshkind world to gain fleeting access. He had instilled his essence in Sky Smith at the moment of her creation, and manipulated events so she would end up with Sarah Jane. He would then awaken Sky's "Trickster" half, and use her to attack Sarah Jane. He would then manifest using Sky and turn Sarah Jane's house into a giant tower that would spread over the world, and he would rule the world. Sky would later destroy him and guard his prison. (The Sarah Jane Companion Volume Three)
 * In The Wedding of Sarah Jane Smith, there were some shots in which small holes in the "eyes" areas of his face could be made out. These were presumably for Paul Marc Davies to see out of.