The scroll of Un-knowledge
It was my first time DMing, only after i played mere two sessions, but i was still however, the most experienced person on the group,we were trying to to a classic prison escape and the party members were:, a monk dragonborn, a rogue doppelganger, a half drow half high elf alchemist and a human mage(needless to say, he wasn’t as creative as the others). They all got stuck in a secret library with an ogre on the next room, and everyone decided to look around for useful things.
(Dragonborn):I look around to see if there any, i dunno, magic scroll i guess?
(Me):Fine i guess
(Me): Hidden in the bookshelf you find mesmerizing scroll, with all the magic thingys floating around it, however, it is Ancient(thinking no one knows Ancient because i’m one of those DM’s)
(Mage): I KNOW ANCIENT!
(Me):Wait, you do?(in my head: dang it, i haven’t thought of anything to put on it)
(Me):Uh, well, roll the d20 to see what you get
(Mage)[OoC]:Yuss, i’m finally gonna have one of those fancy spells, i can finally stop spamming magic arrows!
(Mage)[OoC]:*rolls dice* uh, you know what, i don’t think i wanna read it anymore.
(Me):why is that?*looks to see the dice, it is a nat 1*
(Me): While you read the ancient text, the symbols begin to shamble around you, your eyes gaze through the ancient script, and when you get to the end of the page, you’ve become significantly dumber. You lose 1 Intelligence and 1 Wisdom point.
(Mage):*silently swears at me*
(Everyone else):*Laughs uncontrollably*
On the end of the session after that he decides he didnt like playing a mage and wants to make a new character, so the excuse i give is that he became so dumb that he lied on the floor sucking his thumb while everyone else exited the prison.