"I wasn't aware that Louise Simonson had created Apocalypse since he debuted before she took over. Be that as it may, the Death/Archangel character and related Apocalypse story (which was the first to give Apocalypse a personality beyond the run-of-the-mill criminal mastermind that he was in his first appearance) are definitely very well done. That's one story by L. Simonson I can (and do) appreciate."

Apocalypse was introduced in Wheezy's first issue: #6.

Backstory: Bob Harras asked Wheezy to write a one-off story for X-Factor early around issue 3 because Bob Layton was in jeopardy of not making a deadline. It didn't get used (Layton made his deadline), but a month or two later, she was offered the full-time writing gig when Layton was reassigned. Wheezy came up with the concept for Apocalypse, Butch Guice made the original visual design, and Walt Simonson beefed it up into the visual we saw in print.

Layton's original idea was for the Owl to be X-Factor's big nemesis (the head of the Alliance of Evil). Issue 5's last panel was supposed to be a reveal of the Owl. When Wheezy got the writing assignment for the book, they had Guice change the pencils to depict a character in the shadows who would revealed in the next issue as Apocalypse.

You'll find a lot of this info (and many other X-related behind the scenes stories) directly from the horses' mouths in Tom DeFalco's great book "Comics Creators on the X-Men". It's got the skinny via detailed interviews with Stan, Roy Thomas, Neal Adams, Dave Cockrum, John Byrne, Chris Claremont, Alan Davis, Wheezy, Slivestri, Harras, and others.