Complex sentence: Roxxon Energy Corporation (also known as Roxxon, formerly known as Roxxon Oil Company) is the name of a fictional massive petroleum corporation appearing in comic book stories published by Marvel Comics in the Marvel Universe.
Split into simpler sentences: Roxxon Energy Corporation (also known as Roxxon, formerly known as Roxxon Oil Company) is the name of a fictional massive petroleum corporation. The company appears in comic book stories published by Marvel Comics and exists in that company's shared universe, known as the Marvel Universe.