Cassandra's Curse (3.5e Trait)
From D&D Wiki
You can see the future almost effortlessly. Too bad no one else can know.
Benefit: The character can see the future. What they see is determined by the DM, and as a result is generally regarding some key point in the campaign.
Drawback: To put it simply, while the character knows what's going to happen, they can't tell anyone. If the character attempts to tell anyone, the character's speech will be scrambled, so that anyone hearing him would hear gibberish. Furthermore, if the "prophecy of things to come" is written down then it will appear as scrawlings and doodlings. No spell can counteract this. If the DM is in a particularly sadistic mood, he can allow the character to speak unimpeded, with the condition that no one, and no one at all, will believe the character. Usually, the listeners will gradually become more and more hostile as the character speaks, as the lies get less and less believable.
Roleplaying Ideas: Someone still becoming aware of this trait may be constantly worried or concerned about others whom she sees the future of, this stress will only be magnified by the fact that she is powerless to help them; not that she won't try. More experienced individuals may become bitter or exasperated with people not listening to her, feeling agitated at the fools surrounding her; no matter how many times she warns them they never listen.