Demon King

The Demon King was a character in one version of the pantomime Cinderella which the Thirteenth Doctor went to see with her companions Yasmin Khan and Dan Lewis.

After the Doctor used a link from Spacebook in an effort to bring "the magic of pantomime" to the performance (whose regular version was deemed underwhelming by Dan), the costumed actors were replaced with real versions, "snatched" from other places and realms. This included the Demon King, huge and flaming, who began demanding that all of the audience bow down to him and acting genuinely violent. The Doctor and friends were able to distract him by tricking him into fighting with monstrous alien versions of Cinderella's stepsisters who had also been summoned, then make their way to the understage storage area of the theatre, where the Doctor found a Genie's magic lamp among the props. She was able use one of her three wishes to "wish" her friends safe, resulting in all the pantomime characters being returned to whence they came. (COMIC: It's Behind You!)

Behind the scenes
The Genie mentions that what the Doctor did "snatched" him from another realm. It is not clear if this was specific to the Genie, or if all the pantomime characters in It's Behind You! were summoned from parallel worlds as well, rather than being native to the Doctor's universe.