Baldur's Gate II Walkthrough: Jaheira's Curse Quest
One of Jaheira's character quests in "Baldur's Gate II: Shadows of Amn" will lead to her getting cursed. The only way to cure her curse is to find the man who cursed her and retrieve the lock of hair he stole from her.
Baldur's Gate II Walkthrough -- Getting Jaheira's Curse
If you have Jaheira in your party in Baldur's Gate II, you'll encounter several character-based side quests relating to Jaheira's involvement in the Harpers. One side quest begins when you enter the basement of the Sea's Bounty Tavern, located in the Docks District of Athkatla. (There are two entrances to the tavern; the door leads to the upper floor, where you can find a staircase to the basement, while the outer stairs lead directly to the basement.)
Here, Jaheira will be confronted by a Mr. Proyer. She exposed him as a slaver and tried to get him executed; now he wants his revenge. He apparently use all his remaining money to pay three mages for a curse; he summons them, and they curse Jaheira before disappearing. (If you've visited Proyer's house in the slums prior to this meeting, you'll have seen a note from the mages about the spell -- now you know what that's about.)
The curse gradually decreases Jaheira's stats. Although it's not technically timed, the longer you wait, the lower her stats will get, until she becomes unable to do things like wear her normal armor and wield her normal weapons. You'll notice this effect even more strongly if you rest a few times before curing Jaheira's curse. Having Jaheira not at her best definitely won't help you when you try to fight the mages, so don't go to the Sea's Bounty Tavern unless you know you're ready to get the quest and find Jaheira's cure right away.
Jaheira's Curse -- Where to Go
Baldur's Gate II Walkthrough -- Curing Jaheira's Curse
You'll receive a few clues in-game for how to cure Jaheira's curse. It's not an ordinary curse, so restoration spells and the like won't do. The best option is to talk to the bartender in the Copper Coronet, located in the Slums District. He'll tell you about another Harper who was found dead in the slums -- cursed, just like Jaheira. You can also try to track down the three mages in the Government District, although you won't find much of interest.
Go to Proyer's house, located in the northeast part of the slums. This time he'll be there, along with the three mages that created the curse. This can be a tough fight for lower-level characters. Spells such as Breach and Remove Magic can help you break past their defensive magic. Area effect spells will be more harmful than helpful because of the small size of the room.
After you kill Proyer and the mages, pick up the lock of Jaheira's hair that was used to cast the curse. Although there won't be any immediate effect, Jaheira will be completely cursed 24 hours after retrieving the hair. You don't have to rest to cure Jaheira, but you might as well; that way she'll recover her stats before you go out adventuring again.