Morganna is an evil witch who, centuries ago, used her magical mirror to create an army of darkness that took over the planet, plunging it into the time we know as the Dark Ages. Eventually, Morganna was imprisoned inside her own mirror, until Atomic Roger looked in it, releasing Morganna.
Morganna then proceeded to use the mirror to create an army, using the mirror to unleash the evil mirror opposites of good people, and the good mirror opposites of evil people.
Eventually, Betty, and Penelope's mirror opposite, Good Witch Penelope, defeated Morganna when they learned that the only way to stop her was to get her to look at her own reflection in her magic mirror. They managed to do so, thus sucking Morganna back into the mirror and reversing her magic spells. (Mirror of Morganna)