Depending on the card, it will change your personality by bringing out certain aspects:
Clubs -- Violence. People who have this card become aggressive to the extreme--not just threatening violence, but actually going through with it. They will lash out first and never ask questions at all; the smallest thing could set them off. They won't stop, either; they'll just keep going until they're stopped. People under this effect are noticeably stronger and faster than usual, and are much more twitchy and nervous.
Diamonds -- Passive. These people have become suddenly super-dependent and needy. We're not talking just about co-dependent partner-types, here; these are people who suddenly literally need the approval of everyone, and are willing and desperate to do whatever they can for attention and affection. They want to be loved and admired and noticed, and they may latch onto one person, or to a group of people, or anyone passing by might do to give them their "fix" for attention. They are clingy and afraid, and they need your comfort right now.
Spades -- Sexual. In contrast to the Diamonds, Spades know what they want, and they want you. Even staunchly loyal and devoted lovers might find their partners suddenly wandering, ready and willing to be rule'd no matter what. It's like the love goo, only it's definitely sexual in nature, and inhibitions about the rules are suddenly gone.
Hearts -- Manipulation. "The innocent cruelty of children"--only there's nothing so innocent about this. A lot of it's rather deliberate and rather unkind; it's schadenfreude taken to an extreme level. Offer to do something, only to laugh in their faces when they're grateful. Push someone down the stairs and pretend you saw nothing. It doesn't matter, as long as it's funny.
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Ideally, this plot is a chance for people who have offered certain permissions for trauma/murder/death/violence/etc to have fun and do something with that trauma; HOWEVER, don't forget to be courteous, dudes. Exercise caution, check permission posts,
check this post, and have fun!