Ben 10: Ultimate Alien Episode 1.14 Where the Magic Happens

Gwen, Ben, and Kevin follow Aggregor into a mystical dimension called Legerdomain with the help of Charmcaster, who was born there. They believe Aggregor's target to be the Alpha Rune worn around Addwaitya's neck, which they suspect is a disguised piece of the Map of Infinity. Gwen and Charmcaster work together several times, and the two develop a bond that seems to lead to Charmcaster changing her ways. While the group tries to steal the Alpha Rune, Aggregor uses the distraction to knock Addwaitya out and take it himself. Gwen, Ben, and Kevin escape Legerdomain with Charmcaster's help, but Charmcaster remains behind to free Legerdomain from Addwaitya's rule. Ben promises Gwen that they'll find a way to return and help Charmcaster after Aggregor is defeated.