Reading Blue Diablo by Ann Aguirre gave me a great idea for a curse. One of the characters, Chance, is lucky. Unfortunately, all the bad things that would have happened to him, happen to those around him.
The effects of the curse can range from a mild +2 on a check to avoiding death. Roll to see which of the other members of the party will suffer the consequences. The unfortunate party member will suffer a penalty of equal strength.
With practice the character can learn to control the curse. Instead of taking the bonus, they can take a point of fatigue (or subdual) for the strain of holding it off.
The frequency that the curse strikes is entirely up to you. You are free to pepper an encounter with a range of bonuses or save it for thematic moments.
Armin swung his sword at the dragon's leg. As the beast turned, its tail collided with the cavern wall and a shower of debris fell. He ignored the stones in favor of avoiding the tail, which was sweeping towards him. As he dodged, he heard a cry to his left. Drak, one of his fellow baiters lay on the ground, his leg crushed under a boulder. The boulder had become unstable when the tail collided with the wall, when it fell the tail swept it away from Armin and onto Drak.
Armin cursed. He couldn't control the curse if he didn't know he was in danger. He pulled Drak away from the fighting into a tunnel that was too small for the dragon to follow. Once he made sure Drak was out of harm's way, he returned to the entrance of the tunnel and surveyed the battle. The baiters surrounded the dragon, attempting to force it to reveal the vulnerable areas under the wings so the archers could strike it with arrows laced with dragonsbane. The dragon was old and crafty. It knew how to fight without exposing its weak spots.
As he moved to rejoin the fight, he spotted another fallen comrade. He knelt beside the man to help but the archer was dead, bow still clutched in hand and surrounded by scattered arrows. Suddenly the dragon roared. Armin looked up to see it snatch a man from the ground. As the dragon lifted the warrior to its maw, the raising leg shifted the wing. The underside was exposed!
The curse whispered to him. He could make the shot, but at what cost?