The Magic Mirror is one of several ancient treasures of Daventry once belonging to Legenimor, the First King of Daventry. It has been held in Daventry's hands for almost a millenia. During Edward's reign was stolen by a sorcerer, and hidden away underground. Daventry later found it within the Dragon's Cave, where a green dragon slept, guarding over it. Graham doused the dragon's flames, and the reptilian creature left, leaving the mirror behind.
Sometime later after Graham recovered the mirror, The Father sent a false message through the mirror to Graham to entice him to leave the kingdom to head to Kolyma to find a wife. He intended that Hagatha or Caldaur would kill the king, so that The Father could obtain the crown of Daventry.
A magic mirror was inside of Manannan's safe in KQ3 VGA.
In King's Quest ZZT, Alexander peers into the mirror, and sees Ravenlos mistreating his people. He sees that Ravenlos forces them to fight against innocent people.
In King's Quest ZZT 2, Alexander looks into the mirror not long after the Relentless Army attacked Castle Daventry. He sees a boat, carrying slaves (his parents) to another land.