ThunderCats Roar - Season 1 Episode 15
Study Time

Lion-O never pays attention during Tygras training sessions so when hes asked to do a pop quiz on "sword magic" he accidentally opens a portal to the Astral Plane and releases the Nether Witch. The Witch imprisons Lion-O and Tygra in her nightmare realm and its gonna take a lot of studying to figure out a way home. Wonder if the Astral Plane has any great teachers lying around.