Wanda Maximoff, or the Scarlet Witch, is the daughter of Magneto and sister of Quicksilver. She first appears as a member of the Brotherhood of Mutants in X-men, issue 4. She is one of the most powerful mutants in the Marvel universe with the power to alter reality on a grand scale. In the M-Day story arc, she goes so far as to de-power the vast majority of the world's mutants, leaving just over 200 left on Earth.
The Scarlet Witch's power was originally limited to the creation of hex-spheres, balls of reality-disrupting force capable of causing molecular disturbances in a target's probability field. This power could result in a variety of effects such as spontaneous combustion, or deflection of objects in flight. It wasn't until later that her powers expanded to alter the nature of reality itself. Despite this great level of power, however, she has only undergone a limited amount of training to control it.