Several years ago I ran a Dark Sun campaign where the PCs needed to obtain a vital piece of information for one of their quests for the Veiled Alliance.
The problem was they could only obtain this information from a certain female Templar of Nibenay. So they secretly sent a messenger to her offering some sort of exchange the get the information they wanted.
She agreed to meet them on neutral ground in the desert because the PCs were super paranoid about treachery (rightly so when one deals with Templars).
They bargained with her and she agreed to help them in return for a task. She wanted them to eliminate one of her rival Templars.
They tentatively agreed. However, while the players were all arguing about whether to accept her offer, I casually mentioned: "The Templar, tells you she must cast a spell to divine some more information regarding her rival, do you stop her or say anything?"
"No," the players responded, and then ignored her and continued arguing.
"After her spell is complete, she offers her hand for you all to take. And she asks, 'Are you ready to do this task for me?'
"Do you take her hand and agree?" I ask the group. "Yes, we do," they replied.
"'It is done.' She smiles at you all with an evil grin, 'I have placed a spell upon you,' She announces. 'If you fail to complete this task for me, then you shall die a long, slow death.'"
The look of shock on the players faces was awesome. And then the rules-lawyers started scrambling for their PHB's. But there was nothing they could do. None of them had paid attention to the spell she cast and they had accepted her quest of their own free will. No saving throw.
The players started to draw weapons, but her half-giant bodyguards stepped forward protectively. 'Ah, now that is no way to behave,' she scolded them. 'Remember, I have the information you seek.'"
I was loving this. The PCs had lost all bargaining power. They had to do as she asked and there was no guarantee that she would even uphold her side of the bargain. And of course, she had no intention of doing so. Furthermore, she already knew they were working for the Veiled Alliance and the rival she wanted eliminated was a Veiled Alliance spy. The PCs didn't know this at the time, though. But later when everything became clear, and the sheer depth of her treachery became known to them they were totally beside themselves! It was great!
One PC swore revenge upon her and was totally obsessed with getting her back. He finally did get her back, but it literally cost him his soul in the process. Which is another interesting story in its own right.
Anyway, this is an example of what Geas and Quest spells are really good for. Sealing deals that the PCs make with certain powerful NPCs to guarantee that the PCs do what they promise to do.
