Conviently, I just happened to write this up for my last session. I hope it's helpful!
GEAS
Some promises and words can be broken without an idle thought, but not so with a geas – a geas is a sworn oath made with another bound up into the very deep magic of the world itself. Also known as a Dragon's Promise, a geas can exact a heavy toll if it is broken.
Level: 10
Category: Binding
Component Cost: 0 gp
Market Price: 1,000 gp
Time: 1 minute
Key Skill: Religion
Duration: Until the geas has been violated or lifted.
When words and promises are not enough to ensure someone's good faith, a skilled wizard or priest may invoke a geas. The geas represents a promise between two parties, typically one of mutual protection, honesty or faith. The powerful magics within the world itself acts as a safe holder for the oath, and will punish either party if they break their side of the promise. What this punishment may be is left to the will of the DM, but should be in some way thematically appropriate. A character who has sworn to always tell an ally the truth may find after lying that their tongue withers and they are now mute or that no one believes a word they say. In addition there will always be a visible mark upon the offender, which in some way marks him or her as someone who has broken the geas. The only known way for a geas to be lifted without triggering its awful repercussions is for one party to willingly release the other from it. In a case where multiple people share the same geas, this does not free all of them from its effects at the same time.
A geas cannot be taken up by force – the magic of the geas only takes effect when it is taken up willingly. The magic of the land itself will not accept a false promise or agreement only through threat or intimidation – it must instead come from the participant's hearts. If both parties do not agree to the geas, then the entire geas will not take effect and everyone under its effects is aware of its failure. A character may only be under the effect of one geas at a given time – if a character attempts to agree to a second geas, it will automatically fail even if the character was intent on fulfilling the new oath. Finally, a character is free to offer his own interpretation on how he chooses to accomplish the effects of a geas. If a knight is under a geas to defend a princess who is walking into a dragon's den, he may elect to boldly charge in and face down the dragon or he may elect to knock out the princess and drag her away for her own safety. Both are reasonable examples and neither contradicts the inherent nature of the geas, because of the knight's conviction in both cases that he is fulfilling his oath to protect the princess.