Think of any relatively book, movie, or TV show. There is fanfic about it somewhere. Go type it into a search engine “Your Book of Choice + fanfic”. You’ll get results.
Fan fiction is basically additional material produced by fans of a media title. The internet has made it easier than ever to find fanfic of whatever you want. The newest hottest blockbuster movie, TV show, or book tends to be the easiest to find. These are of course produced without the original creator’s consent and often take characters in directions that weren’t intended. New characters get added. Old annoying characters get killed off in the ways fans dreamed about and never got. Characters finally hook up with the person they’ve been making gooshy eyes at for the entire series . Bromance blossoms into actual homosexual romance. There’s sex scenes that would never get past censors.
Due to the overflowing abundance of fan fiction on the internet, it often seems like a modern online only phenomenon. But it’s hardly new. Mainstream publishers snap up fan fiction of books that have fallen out of copyright. There’s at least 20 books put out by mainstream presses that are spin off’s of Jane Austin’s Pride and Prejudice that are sequels, prequels, retellings from a different perspective, or just downright weird retellings. Pride and Prejudice and Zombies is one of the best known current ones.