I am running a campaign in which I expect the PCs will encounter a nothic. I want to use the nothic's Weird Insight ability to have it learn the characters' fears or flaws and use that information to telepathically speak discouraging things to the minds of the PCs who failed to avoid the Weird Insight.
For example, one of our players is a sorcerer princess who fears that she will never fill the shoes of her mother, the queen. The nothic might say "You are far too naive and irresponsible to rule. Though she has erected a loving facade, your inability to control your powers brings nothing but shame and disgust to your mother. If I were to allow you to leave this place alive and you should someday take the throne, you will bring your kingdom nothing but ruin and misery. Fortunately for your subjects, I will not."
I want to then give the PCs an opportunity to make a Wisdom saving throw—if they succeed the saving throw, they are unaffected by the nothic's taunts. Otherwise, I want them to have some kind of mechanical disadvantage representing the nothic's success at discouraging them and lowering their confidence. It doesn't seem like the PHB lists any kind of condition that meets this criteria—is there such a condition in another rulebook? Are there any official rules or guidelines regarding the creation of custom conditions?