Desolation

Only being described with either a symbol or a warning, Desolation was the closest to a name the planet has. Once ruled by the Stenza, it was used to by the race to create and test a variety of weapons, including poisons.

Eventually the planet's ecosystem was completely destroyed and it became a barren wasteland. The water itself became full of flesh-eating microbes and the planet itself fell out of its orbit. The planet was used as the final stage of the Rally of the Twelve Galaxies. During this event, the Thirteenth Doctor, Graham O'Brien, Ryan Sinclair and Yasmin Khan visited the planet to search for the TARDIS. (TV: The Ghost Monument)