Guy Stauber, who has worked with Marvel, Disney and DC Comics, has produced some stunning artwork for this and the post talks about this and shares some of the images. But going on to fiction, where do legends and myths come in to that?
One aspect of world building is to create what your society believes/honours and what its literature would be about. All worlds should have some form of literature it consists important. A fictional world has to have background and substance and inventing legends and myths your characters would read/know about adds depth to your creation. They will help make your world seem more real. Why?
Because those legends and myths would resonate with readers as each of our societies has its own variants here. Overall we can all identify with a world where a legendary hero fights battles and wins (think Beowulf to Doctor Who!). A fictional world without such background would seem as it if had something missing.
And legends and myths can inspire your characters and give them a sound reason (at least from their viewpoint) for taking the actions they do, again adding depth to your story. So invent away! Most of the details don't have to actually go in the story, just enough so your reader can work out what the legend/myth would be and can see why they would be important to your characters. Of course, if lucky, you could produce those legends/myths as separate stories/books to your main ones. I'm thinking of things like J.K. Rowling's Fantasic Beasts and Where to Find Them here.