The Tower in the Woods

It actually happened. In fairly close detail to how the story works, if you account for genre embellishments. Every elementary-age school kid, age six to eleven, knows the story, passed on from class to class, moving into legend.
The story, in its most common format, is something like a portal fantasy, except completely plausible because everyone knows that alien portals are a thing. Most people will never in their life go through one, but it is somewhat tacitly accepted that they exist.
The basic idea of the story is that a group of kids wandered in the woods, fell through a portal, had adventures of varying kinds, and eventually ended up facing off against a dangerous killer who lived in a massive gray stone tower. You can see it on rainy days, if you look up high enough. Supposedly.
The kids, who everyone eventually admits were probably teenagers since a romance subplot is also quite a common factor, face and kill this man in varying gruesome fashions, and then return home only to get in massive amounts of trouble.
E/N: This is one of the most generic, cut down versions of the story I've ever seen. And I've been to enough elementary schools to know that this is selling the story short. It's the kind of thing most adults don't want kids talking about because it gets gross and dangerous and includes real evil. And also children killing someone. But the story exists and gets passed on, and maybe someday it gets turned into Ring Around the Rosy, where you have to really search to find the core meaning. But for now, it's only about fifty years old so the details are still a little too plausible.
The reality of the story involves one of the first Academics Club chapters ever, and the alien serial killer who went after superpowered children. And they did kill him, after going through a portal to his homeworld, where they lived in massive floating towers.