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. 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. 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 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 reappeared as the antagonist of NOTVALID: Zellin's Nightmare World, a game printed in Doctor Who The Official Annual 2021, where he displayed the ability to alter one's memories of one's own dreams. He set a challenge to the "player character" to identify the seven differences between two memories of the same dream, claiming that if they failed the test he would "live in [their] nightmares forever".