Bound in Word and Wonder

In mythology, fairy tales, and fantasy stories, the idea of a magically binding contract occasionally shows up, generally when dealing with powerful supernatural beings. An old witch sets a strange and impossible task for a child, but must let him go free when he somehow accomplishes it. A man bargains with the devil, trading away his hope for an afterlife in exchange for wealth and power in this life. A wizened little gnome promises to return the stolen child only if the mother can guess the gnome's name. However it works, there is something magical about these kinds of arbitrary mystically enforced rules. They reinforce the meaningfulness of even trivial interactions, and they give an air of storybook wonder to what could otherwise be dull tasks.

The idea of binding oaths and bargains is explored in many games. As far as I know, White Wolf's Changeling: The Lost is the first to really codify these agreements into an actual detailed system, though I'm sure others have come before. As such, the system I'm proposing here owes a lot to that game. My system is generic: the specific details will need to be filled in to fit whatever system it will be used with -- in my case, most like Ars Magica, Exalted, Changeling, and GURPS.

The Components of an Oath
Each oath is constructed from several components. Each component has a rating describing how significant it is, with a number associated with each rating:

• Trivial (0)
• Minor (1)
• Moderate (2)
• Major (3)
• Mythic (5)

Each component is also described as 'positive', 'negative', or 'neutral.' Positive components add to the value of the oath, negative components subtract from it, and neutral components don't actually affect the oath's value at all. Oaths should generally sum to a total value of zero, excepting special rules to the contrary (some games might allow fairy lords to have positive-value oaths, or require oaths with demons to have a net negative value at some level, etc.)

Personal, Dual, and Group Oaths
A personal oath is the simplest to build; you just set the components together, make sure they add up to zero, and you're done. Many systems and settings either won't support personal oaths or will penalize them somehow, though, since they're lacking the aspect of interaction that is most of the purpose of magical oaths.
A dual oath is probably the standard; an agreement between two individuals. To build it, you design each individual's side of the oath as a separate oath and make sure the durations match and there are no contradictions. When one character breaks the oath, the other one involved has his end of the oath nullified as well but does not suffer any punishment for breaking an oath (since he didn't break it.)
A group oath works like a dual oath, except that when someone breaks their end of the oath, provided there are still at least two people sworn to it and the oath still makes sense, the oath is not nullified for the remaining characters. For instance, if a group of characters swear an alliance to each other and one turns traitor, the traitor suffers the punishment and loses the benefits of the oath, but the rest of the group is still bound to the alliance to one another.

Duration
Every oath requires a single Duration component, defining how long it lasts. Duration has a variable value; sometimes it is considered positive, other times negative, and sometimes neutral. This depends on the other components of the oath. If even one component states that it treats Duration as positive, it is a positive component. If no components treat duration as positive and at least one describes it as negative, it is negative. Otherwise, it is neutral.

• Trivial Durations last no longer than a quarter of a day. Examples include: 'until sunset' (on an oath sworn at noon), 'an hour', 'the length of a sunrise' (about two minutes; the time it takes the sun to go from showing only a sliver to fully crossing the horizon), etc.
• Minor Durations last up to a week. Examples include: 'until Sunday', 'when the sun has twice risen and fallen'.
• Moderate Durations last for a long while, but significantly less than a year: 'for an entire lunar cycle', 'until the next solstice', 'forty days and forty nights', etc.
  
• Major Durations last for long but finite spans of time. Examples include: 'until your child is born', 'for a year and a day', or even 'in a decade's time'.
  
• Mythic Durations can be truly impressive: 'until the day you die', 'unto the seventh generation', 'for three centuries', etc. At this level there's no real limit to the duration, and oaths can even be set to last forever (provided nobody breaks the oath, of course.)

(part II, detailing 'Challenges', to come later)