From Wikipedia:
Louise Simonson (b.1946) is an American comic book writer and editor. She is best known for her work on comic book titles such as Conan the Barbarian, Power Pack, X-Factor, New Mutants, Superman: The Man of Steel, and Steel.
In 1986 Bob Layton, writer of the X-Men spin-off X-Factor, was running late on a deadline, and Simonson was called in to write a fill-in issue of X-Factor. This story was never published, since Layton ultimately turned his story in on time, but while writing it Simonson found herself inspired by the characters, to the point where she brought a list of her ideas to editor Bob Harras in the hopes that Layton might use them for the series.
Instead, Layton ended up dropping X-Factor shortly after, and at Chris Claremont and Ann Nocenti's suggestion, Harras chose Simonson as his replacement. In #6, her first issue, she and artist Jackson Guice introduced Apocalypse, a character who would go on to make repeated appearances in the X-Men franchise. From #10 of the title, she was joined by her husband, Walt Simonson, on pencils.