Wasn’t there an episode where the creature fed on fear. And they finally got rid of it by getting everyone to laugh.
Fear, anger and hate “Day of the Dove”. One of the more entertaining episodes both for the story (which is one of the better ones) and the gaffs (like Klingon make-up sticking to the wall during one of the fight scenes). Later ripped off for the painfully boring Farscape episode “Crackers don’t Matter”.