A guardian was the final survivor of the super race of Uxarieans and the overseer of the Uxariean city. When the Third Doctor and Jo Grant were to be sacrificed, he let them go on one condition: that they never come back to the city.
When the Master forced the Doctor back to the city to find the Doomsday Weapon, the Doctor met the Guardian again. The Guardian allowed the Doctor to press the self-destruct button for the Doomsday weapon, rather then let it fall into the hands of the Master. The resulting explosion killed everyone in the city, including the Guardian. (TV: Colony in Space)