Complex sentence: Jack Frost is the name of two unrelated fictional characters in comic books published by the Marvel Comics, the first of them published by Marvel's 1940s forerunner, Timely Comics, during the period fans and historians call the Golden Age of comic books.
Split into simpler sentences: Jack Frost is the name of two unrelated fictional characters in comic books published by the Marvel Comics. The first Jack Frost was published by Marvel's 1940s forerunner Timely Comics during the period fans and historians call the Golden Age of comic books.