ThunderCats Roar (2020)

S1E15 Study Time

Lion-O is distracted during Tygra's training and, when prompted to take a quiz on "sword magic," he inadvertently opens a portal to the Astral Plane, setting free the Nether Witch. Soon, she traps both Lion-O and Tygra in her terrifying nightmare world, leaving them with the daunting task of studying their way back home. They can only hope that the Astral Plane has some wise mentors to help them out.