The Elixir of Life was a product of the Sacred Flame, used by the Sisterhood of Karn to give them extended life.
It could also be used to heal serious injuries. Time Lords occasionally used it when undergoing a difficult regeneration. The Sisterhood used it to heal the Fourth Doctor after his mindbending battle with Morbius. With special preparation, a variant of the Elixir could also trigger a Time Lord's regeneration and influence both the physical and personality traits of the new self. After the Eighth Doctor crashed on Karn during the Time War, the Sisterhood provided him with a prepared Elixir that allowed him to regenerate into the War Doctor. (WC: The Night of the Doctor)
The Elixir was produced on Karn as water from underground springs passed through the rocks heated by the Sacred Flame (which was prone to the generation of sooty residue) directly below them. The rocks contained rare minerals and compounds which were released into the water when heated. A goblet was placed below the flame to catch the Elixir. (TV: The Brain of Morbius)
The Elixir was also part of some invasion plans, as well as the Key to Power and the Tree of Knowledge. These plans were stolen by the Fifth Doctor. (GAME: The First Adventure).
