The Magic Mirror is one of the gifts Morgana received for her birthday. Gwen said it was from King Gormause, but in reality it was a present from her sister Morgause which contained a secret message: "Sister, come to the Darkling Woods at midnight, Morgause." Morgana must have been expecting this gift, because she instantly understood that King Gorgmause was in fact her sister (The Crystal Cave).
Morgana reads the mirror