She is accidently summoned by, and later possesses, the movie’s protagonist Susan. The Evil Queen uses the beautiful possessed blonde to seduce and murder men so she can sustain herself with their blood.
Throughout the movie, Susan kills men and returns to the mirror where the Evil Queen resides, she feeds the Queen with blood through the mirror and helps her become stronger.
At the end of the movie, a man called Beko frees Susan from her possessed state by using some kind of magic. He then manages to keep the Queen trapped within the mirror and seemingly kills her too, as she is last seen shattering into pieces as if she was made of glass.