Karn

Karn was once a colony planet of Gallifrey. It was later the home of the powerful Sisterhood of Karn and bolt-hole of the Time Lord criminal Morbius.

Characteristics
Following the defeat of Morbius, Karn had become a burned-out and exhausted planet, a barren and rocky wasteland with many ruins and abandoned industrial plantations. It was littered with the wreckage of crashed spaceships from many worlds, brought down by the Sisterhood's telekinetic powers out of fear of theft of the Elixir of Life.

The Doctor stated that his birth had taken place a few billion miles away from Karn, by implication placing it near Gallifrey. (TV: The Brain of Morbius)

Karn was the first place to pop into the Eleventh Doctor's head when he tried to think of the rainiest, wettest, stormiest place he had ever been so he could compare it to the storms created by the Cybermen in Klimtenburg, but then he decided Margate made for a better comparison than Karn. (PROSE: Plague of the Cybermen)

Prior to the Last Great Time War
Karn was originally a colony of the old Gallifreyan Empire in the days before Rassilon. The Sisterhood of Karn were a remnant of the Pythias who once ruled Gallifrey before being expelled by the triumvirate of Rassilon, Omega, and the Other. (PROSE: Cat's Cradle: Time's Crucible)

During the Rassilon Era, the Sisterhood guarded the Sacred Flame. The Sacred Flame was used to distill the Elixir of Life, which could aid a Time Lord who experienced difficulty with regeneration. (TV: The Brain of Morbius)

Morbius, a renegade Time Lord, in his bid for galactic conquest, came to Karn with a mercenary army. The Time Lords cornered him there, defeated, tried and supposedly executed him. Mehendri Solon, a human surgeon belonging to the Cult of Morbius, had managed to secretly remove Morbius' brain. He preserved it, and vowed to resurrect him. Solon settled on the planet in a disused hydrogen plant and sought a way to make Morbius live again. He used the body parts from dead crew members of different species' crashed ships to create a crude, grotesque new body for him.

The Fourth Doctor and his companion Sarah Jane Smith stumbled across this plot after the Time Lords redirected the Doctor's TARDIS to the planet. The Doctor defeated Morbius, killing Solon in the process. (TV: The Brain of Morbius)

During the Last Great Time War
The Sisterhood remained on Karn. Their leader, Ohila, anticipated the Doctor's return in hopes he could stop the war then raging across creation. The Eighth Doctor, responding to an emergency distress signal from a gunship that had been damaged in orbit, attempted to rescue the sole remaining crewmember, a woman named Cass. She refused to board the TARDIS when she realized he was a Time Lord, declaring that they had become as vicious as the Daleks. The Doctor, unwilling to leave her to die, was killed when the ship crash-landed, still trying to convince her to leave. The Sisterhood restored him to life for only a few minutes, allowing him to regenerate into his next incarnation. (TV: The Night of the Doctor)

Alternative timelines
In a negated timeline, Cristophe Zarodnix purchased Karn from the Sisterhood. From there he used a fragment of Morbius' brain and genes of Straxus to recreate Morbius, who from there ruled an empire. When the Eighth Doctor turned off Morbius' Stellar Manipulator the Time Lords prevented this Empire from forming, however Morbius fell with the Doctor into a chasm, which still happened despite the timeline being prevented. (AUDIO: Sisters of the Flame/Vengeance of Morbius) However the Doctor was saved by the Sisterhood and transported to Orbis. (AUDIO: Orbis)

Origins
Karn originated in a theatrical play, Doctor Who and the Daleks in Seven Keys to Doomsday, written by Terrance Dicks. Dicks re-used the name in the script for The Brain of Morbius. The play featured an alternative Fourth Doctor.

This wiki considers neither the play, nor its audio adaptation by Big Finish, a valid source, and therefore sees The Brain of Morbius as the first appearance of Karn in the DWU.