Zellin

Zellin was an Eternal, (PROSE: The Guide to the Dark Times) and the lover of Rakaya. He appeared as an old man with tattoos on his head. He was able to manipulate the atoms of his body and used this ability to remove his fingers from his hands and remotely plant them into people's ears to induce nightmares, which entertained him. He was able to regrow his fingers after doing this, thus allowing him to leave his fingers implanted in his victims. He was even able to manifest people's fears as reality, such as the Chagaskas. Both he and Rakaya 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, he and Rakaya 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. They eventually became aware of Zellin and Rakaya's influence and imprisoned Rakaya, but Zellin managed to escape before he could be too. He kidnapped Tibo, one of Ryan Sinclair's friends, among others, in order to use their nightmares as sustenance for Rakaya during her imprisonment.

After luring the Thirteenth Doctor to the monitoring station for Rakaya's prison, Zellin was able to trick the Doctor into freeing Rakaya. The Doctor eventually managed to trap both of them inside of Rakaya's prison, now to be tormented by the nightmares of the people they formerly tortured. (TV: Can You Hear Me?)

Behind the scenes
Zellin shares many similarities with the Nightmare Man from The Sarah Jane Adventures story of the same name, in that both are extra-dimensional beings who feed off of humans' nightmares.