Serenity was a colony of the Traken Union located at the edge of the Mutter's Spiral, (PROSE: Cold Fusion) or the Silver Devastation. (AUDIO: Cold Fusion) It was used by the Guild of Adjudicators as a model of peaceful and just society. Until she learned of the existence of Serenity, Nyssa believed herself to be the last of her race, as Traken was destroyed by a wave of entropy in 1981. They were spared from the catastrophe by the Source, which was created there, before it was destroyed as well. (PROSE: Cold Fusion, TV: Logopolis)
Following the fall of the Traken Union in 1981, it formed the Benign Union. Melkurs were present on Serenity when the Sixth Doctor and Peri Brown visited it. (AUDIO: The Guardians of Prophecy)