S1·E33 Dimension Doom

Wiz-Ra, Mumm-Ra's enemy from long ago, comes back every 7000 years through a portal that just happens to be in Cheetara's bedroom. Wiz-Ra wears a magical helmet that lets him control minds. When the time arrives and Wiz-Ra comes through the portal, Mumm-Ra steals the helmet. Cheetara helps Wiz-Ra get the helmet back and free the ThunderCats from its spell.