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. (TV: The Brain of Morbius)

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)

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 managed to defeat 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. A gunship containing the Eighth Doctor and the ship's sole remaining crew member Cass crash-landed. Cass had rejected an offer to leave the gunship onboard his TARDIS because he was a Time Lord, after the race had grown equally as vicious Daleks, which left the Doctor little other choice than to stay on board and go down with her, not willing to abandon someone in jeopardy. The Doctor could not rescue Cass in any way and died alongside her, but the Sisterhood restored him to life for only a few minutes, so that he might 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.(AUDIO: Sisters of the Flame)

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.