X-Men: How Apocalypse Turned Gambit Into His Horseman

Here's how Gambit of the X-Men ended up (briefly) turning into a servant of Apocalypse as the Horseman of Death.

X-Men: Age of Apocalypse Omnibus by Mark Waid

Horsemen of Apocalypse (Earth-616), Marvel Database

The 25 Strongest Horsemen of Apocalypse

Who are the 4 horse-men you want in this game : r/future_fight

4 Times Good X-Men Turned Bad

X-Men: The History Of Apocalypse's Four Horsemen

X-Men: Apocalypse's Four Horsemen Explain Their Roles

Gentlemen of Leisure: To Better Know a Villain: The Four Horsemen of Apocalypse

Horsemen of Apocalypse (Earth-616), Marvel Database