After being transformed into Pestilence, Plague, War, and Famine battled X-Factor in Central Park and were defeated by Iceman. After the fourth Horseman, Death, was revealed, Apocalypse had the Horsemen battle each other, to find the leader and Death beat them all.
After Apocalypse teleported X-Factor on his Celestial Ship, they battled the Horsemen again and, after their defeat, the Horsemen were sent into Manhattan to incite chaos. X-Factor and the Power Pack battled the Horsemen and Lightspeed knocked Plague off her flying steed, whereupon she fell to her death.
Plague was a Morlock. She appeared as a woman wanting a cure and was transformed into the Horseman Pestilence.