ThunderCats S01E39 - Monkian's Bargain

Monkian goes to Mumm-ra and agrees to give him anything if Mumm-ra will make him powerful enough to rule Third Earth. Mumm-ra agrees cleverly, and gives him Power Spheres which make him super strong and give him super weapons. He freezes all the Thundercats, then defeats Lion-O and takes the Sword of Omens back to Mumm-ra. Mumm-ra takes it, and then says that Monkian's part of the bargain will be that he can keep the spheres, but he has to stay in Mumm-ra's pyramid for all eternity.

The walls start to surround Monkian, and every time he breaks them down, a new set of walls shows up. While Monkian and Mumm-ra are arguing, Lion-O retrieves the sword and frees the other Thundercats. Monkian gives up his power to be free again.