Ranskoor Av Kolos was a planet, whose name roughly translated to "Disintegrator of the Soul". It was one of three planets where the Ux have been found. The planet transmitted violent psychotropic waves throughout its atmosphere that caused negative effects such as memory loss, reality distortions, and mood changes.
In 2018, Tzim-Sha was teleported to Ranskoor Av Kolos after his Stenza recall device was corrupted by the Thirteenth Doctor. There, he encountered Andinio and Delph of the Ux, who believed him to be the Creator of their religion. Under his direction, the Ux created a massive floating building that combined their powers with Stenza technology to form a weapon that could snatch other planets out of space. These planets would be miniaturised and kept in stasis inside crystals.
By 5425, Tzim-Sha had used his weapon to steal five planets, which destroyed all life upon them. In response to these atrocities, the Congress of the Nine Planets sent their fleets to Ranskoor Av Kolos to stop Tzim-Sha, but they were all brought down by the power of the Ux. The Thirteenth Doctor, responding to their distress signals, discovered Tzim-Sha's activities and managed to stop him from taking Earth and destroying all life there are well out of revenge for his defeat.
With the aid of the Ux and her TARDIS, she was able to restore the five planets to their proper places. In the aftermath, the Ux decided to abandon Ranskoor Av Kolos to learn more about the universe. (TV: The Battle of Ranskoor Av Kolos)