Puella Magi Kazumi Magica, Vol. 3: The Innocent Malice — Magica Quartet

As her teammates look on in horror, Niko succumbs to the taint that has overwhelmed her soul gem and she transforms into a terrifying witch. But there is little time for the Pleiades Saints to grieve-this new witch has gone on the attack! Left with no alternative, Mirai strikes down her friend to save the rest of the girls. Is there any power that can save the Magical Girls from their fate?