Karn

A planet somewhat near Gallifrey seen in the episode The Brain of Morbius. It is notable for being the home of the Sisterhood of Karn, an order of nearly immortal women who guard the Sacred Flame. The Sacred Flame is used to distill the Elixir of Life, which can aid a Time Lord who is experiencing difficulty with regeneration.

The planet, or at least portion of it revealed during the events of The Brain of Morbius, appears to be a barren, rocky wasteland, littered with the wreckage of crashed spaceships.

Karn is also notable as the final refuge of the Time Lord criminal Morbius, whose disembodied brain was transplanted into a patchwork body by Dr. Solon. His return to power was stopped by the Fourth Doctor, Sarah Jane Smith, and the Sisterhood.

The Expanded Universe
There are hints in the Virgin New Adventure novels that Karn was originally a colony of the old Gallifreyan Empire which existed in the days before Rassilon. They also suggest that the Sisterhood are a remnant of the Pythias which once ruled Gallifrey before being ousted by the triumvirate of Rassilon, Omega, and the Other.