Serenity (Cold Fusion)

Serenity was a colony of the Traken Union located at the edge of the Mutter's Spiral. 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. (MA: Cold Fusion / DW: Logopolis)

Melkurs were present on Serenity when the Sixth Doctor and Peri visited it. (BFA: The Guardians of Prophecy)