Jadis has become so powerful controlling Narnia that she decides to find other worlds to conquer. She looks in her crystal ball for other worlds to take over and finds Medieval Europe, 14th century. So she uses a spell to travel to this place and when she gets there, sees a magnificent castle (King Stephen's).
She sneaks into the highest tower and sees Princess Aurora about to prick her finger on the spindle of Maleficent's spinning wheel. She figures that she is the king's daughter and that she could use the princess in order to get what she wants from the king. So right before Aurora can prick her finger, Jadis uses her wand to turn her into stone (let's just pretend for now that Jadis's magic is not useless in other worlds). Her plan is to use Aurora to have King Stephen forfeit the kingdom in return for his daughter back. Needless to say, this doesn't sit well with Maleficent, who remember is in the tower right then.
Maleficent makes herself visible and gives Jadis the chance to return to where she came from safe and sound if she turns Aurora back into flesh. Jadis refuses and a battle ensues between the two witches.
Who wins? It's movie versions of both characters and remember: we're pretending just for the time being that Jadis is as powerful in other worlds as she is in Narnia.