Complex sentence: Valkyrie is a fictional superhero created by Roy Thomas and John Buscema that appears in comic books published by Marvel Comics, based on the Norse mythological figure Brynhildr.
Split into simpler sentences: Valkyrie is a fictional superhero appearing in American comic books published by Marvel Comics. The character was created by Roy Thomas and John Buscema, based on the Norse mythological figure Brynhildr.