The Trickster

The Trickster was an extra-dimensional alien, formerly part of an extra dimensional group of evil 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 his attempts to draw power from the chaos he creates and thus enter the world in his full power. Frequently, the Trickster came up against former time traveller Sarah Jane Smith, eventually becoming her greatest and most recurring enemy. He was also likely the leader and creator of the Trickster's Brigade.

Biology
The Trickster 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. (SJA: Whatever Happened to Sarah Jane?) He had power over people, but only with their consent. As such, he tended to go after the dying, giving them life in return for servitude. (SJA: 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. (SJA: Whatever Happened to Sarah Jane?, The Temptation of Sarah Jane Smith) When he appeared at Sarah Jane's wedding he wore the same outfit, but the colour was entirely changed to white, resuming his more customary black only while apart from Sarah. He wore white so as Peter Dalton believed he was an angel, in order to make him believe he was there to help and protect him and Sarah Jane. (SJA: The Wedding of Sarah Jane Smith)

Biography
Not much was known about the Trickster's origin, other than the fact that he had parted ways with the group known as the Pantheon of Discord. All that was truly recorded was his attempts to influence history. (SJA: 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 though only if he gave him his consent. Krislok agreed, and the Trickster rescued him. However, in exchange for saving him, the Trickster forced Krislok to become his slave. (SJA: The Temptation of Sarah Jane Smith)

Later on, to satisfy his need for chaos, the Trickster found one main event in 2008, which if it happened would have caused destruction on Earth and wiped out Humanity, who would later become a major species in the Universe. This event was that of a meteorite crashing into the Earth, undetected by military radars. There were many other times when life on Earth would have been threatened by alien life, however the meteorite crash was one of those few which had no purpose, it just being 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. (SJA: Whatever Happened to Sarah Jane?)

First Attack
In the original time line, the impact would have been stopped by Sarah Jane Smith and Mr. Smith. To stop this, the Trickster removed Sarah Jane Smith from Earth's time line, influencing events so she died in the place of Andrea Yates (who should have died in the original time line) 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, one night Maria Jackson held an alien puzzle box, protecting her from the effects and allowing her to remember the original time line. She reminded Andrea Yates of the day Sarah Jane died, causing Andrea trauma. The Trickster sent his Graske servant Krislok to remove Maria and later removed her from history, however her father found and held the cube at the point of 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, however Alan foiled the plan and trapped the Graske.

He discovered a device on Krislok which summoned Maria back to existence from Limbo, and the two headed to Sarah Jane's house (in this time line, Andrea's house) where Sarah Jane materialized in a mirror. She explained that while Andrea was alive, Sarah Jane couldn't be. Andrea, realizing that she had to die (either by falling off a pier in 1964 or in the soon-to-be meteorite crash), went back on her deal with the Trickster. The time line was back to normal, Sarah Jane and Mr. Smith deflected the incoming meteorite and the Trickster was defeated. Had his plan succeed, the Trickster revealed he also wished to use Sarah Jane to locate the Doctor and remove him from time, which would have resulted in an untold amount of chaos. (SJA: Whatever Happened to Sarah Jane?)

Revenge
The Trickster decided to get revenge on Sarah Jane, opening a time portal to 1951, where she decided to meet her parents as she never had met them. Sarah Jane and Luke deactivated her father's car, thus saving her parents from death. By creating this weakness in time 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 where 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 to manifest, and her parents willingly sacrificed themselves to correct the time line. (SJA: The Temptation of Sarah Jane Smith)

An Uninvited Wedding Guest
When Peter Dalton fell on a staircase and was about to die, the Trickster came 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. The Doctor somehow learned of this and interrupted the wedding, demanding it to 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 there forever. If married, Sarah Jane would stop fighting aliens, thus would cause chaos.

The Doctor's TARDIS tried to get into the time loop, and the Doctor was finally 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. Both he and Clyde were sent back in the reception room as the Doctor arrived. When Peter learned of the full story and the consequences of his action, he went back on his deal and threw his ring into the Trickster, defeating him. (SJA: The Wedding of Sarah Jane Smith)

Other Influences
Some time in the future there existed members of the Trickster's Brigade. One known member was a Fortune Teller on Shan Shen who used a beetle-like creature, causing Donna Noble to experience an alternate time line in which the Doctor died. (DW: Turn Left)

By the 26th century some individuals worshipped the Trickster as the "God of Randomness". (NA: Love and War)

Behind the scenes

 * The Trickster is similar in nature to the Black Guardian although, as yet, no formal connection has been made in the Doctor Who Universe. The Trickster's "agreements" with his victims just prior to their imminent deaths are very similar to the bargain struck between the Black Guardian and Vislor Turlough. Both the Black Guardian and the Trickster have been heard to use the phrase, "Waking or sleeping, I am always with you."
 * Initially the character's name was only given to the being by Alan Jackson as a description of him and, as such, the being 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 as "The Trickster" by the Doctor in Turn Left and The Wedding of Sarah Jane Smith when the Doctor called him such by name.
 * The Doctor refers to the Trickster as being "an eternal exile" (i.e. always an exile); it is unknown whether or not he was saying the Trickster was in fact, an exile of the Eternals.