Rakaya

Rakaya was an immortal being from another universe, and the lover of Zellin. She appeared as a woman with dark complexion and platinum blonde hair. She was allegedly able to manipulate matter. Both she and Zellin were able to teleport across both time and space, even able to manifest inside the TARDIS. When teleporting, they appeared as shadowy, smoke-like clouds.

At some point in the distant past, she and Zellin found two inhabited planets in the same solar system and made a bet as to who could destroy a planet first to amuse themselves. The inhabitants of the planets went to war with their own people and eventually each other. The inhabitants of the planets eventually became aware of Zellin and Rakaya's influence and managed to imprison Rakaya, but Zellin managed to escape before he could be imprisoned. During her captivity, Zellin fed her human nightmares to keep her sane.

She mentally contacted Graham O'Brien, companion of the Thirteenth Doctor, seeking for help. The Doctor and her team were fooled about her true nature and the Time Lord herself was tricked by Zellin into freeing her. Eventually, the Doctor managed to imprison Rakaya again, this time with Zellin, the pair of them to be eternally menaced by the same nightmares they had been feeding on, such as the Chagaskas. (TV: Can You Hear Me?)