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 by the race to create and test a variety of weapons, including poisons.

The Stenza kidnapped and tortured a group of soldiers to create new means of destruction, and they used Desolation as a testing ground.

Eventually the planet's ecosystem was completely destroyed, and it became a barren wasteland. The water became full of flesh-eating microbes, and the planet itself fell out of its orbit. The previous inhabitants of the planet were killed, leaving behind their deadly creations, which included the Remnants and the SniperBots.

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, which manifested briefly on the planet every thousand solar cycles due to it phasing in and out after the damage from the Twelfth Doctor's regeneration into the Thirteenth Doctor. It became known as The Ghost Monument as a result. With help from the Doctor and her companions, the two finalists, Angstrom and Epzo, both won the race and were teleported away from the planet by Ilin. The Doctor was able to stabilise her TARDIS and escape Desolation. (TV: The Ghost Monument)