The king or some important noble's daughter was drawn into an evil religion, which involves drugs, orgies and other corrupting influences. He wants her back, but she doesn't want to come, enjoying it as she is. In reality, she is being kept around to encourage his good behaviour, but they won't hesitate to kill her ( even though she doesn't know it ). If the party attempts to get her back, she will use the first opportunity to betray them or escape, but won't do this until the worst possible moment. She could leave a few tokens on the trail which the PC's would miss. The PC's have no reason to suspect her. The best way to portray this is to have the princess, etc being given the best possible quarters for her "prison", which she doesn't leave. She could also have gained formidable spell powers in a few months of casting spells within the ethos. This will annoy the players if they need to take her back to get their reward, especially since they have made another enemy in the form of the corrupting religion. The princess can deliberately deceive the players into believing her innocence. A few red herrings, such as the evil church actually sending out assassins to kill her, instead of recovering her ( which of course, she doesn't believe until it is too late after she manages to escape and go back. )
The reason for this is that the king wasn't behaving himself by sending adventurers after her, especially if the King has just blown it before by standing up to the evil church because he has come to the decision that he might have to take the risk and they are about to carry out their threat because of it, which makes time very crucial.
The best way to give them a time limit is to set a special event upon which the princess is to be unknowingly sacrificed. If the PC's fail to pull it off in time, they get no money for their trouble, make an enemy of both the king and evil church, as well as probably being suckered with a powerful curse.