Here's a more explicit version of the re-vamped Wish spell I was kicking around in the last post.
Wish Pact (9th Level Spell)
Duration: 1 hour. Range: 6".
A special form of Geas which binds two beings in a master-servant relationship for either a fixed period (such as a year and a day,) a specific condition (the master dies, or the sun sets in the east,) or a specific number of services (a single wish, three wishes, etc.) It's possible to combine restrictions, such as granting a single wish every week for a year and a day.
The servant can't be bound without consent to all conditions in the supernaturally-binding agreement, and the master must accept the offer. Normally, it is the servant who casts the spell, agreeing to be bound by a geas until the conditions are fulfilled in exchange for something the master can offer, such as release from a supernatural prison.
The duration and range given are for the the geas part of the wish pact; when the servant casts the spell, offering to serve a master, negotiation can last up to one hour; if no agreement is reached by then, neither party is bound.
The benefit of the spell for the master, aside from the obvious, is that the servant cannot harm the master for the duration of the agreement, except in accordance with the master's demands. Even then, if the master hasn't broken the agreement and still has wishes remaining, the servant cannot take away the master's right to further wishes. Thus, if a master accidentally says "I wish I were dead!" as the first of three wishes, the servant can't kill the master, but could transform the master into an undead monster. Once all wishes have been granted, however, the freed servant can do anything it desires to the former master unless the agreement included some form of protection clause covering exactly that situation.
The benefit of the Wish Pact spell for the servant is the ability to cast any clerical, magical, or other spell known to the world, with a one-round casting time and no need for special ingredients (other than the normal target of a spell, such as needing water to cast Part Water.) Secret spells that aren't on the common spell lists or haven't been invented yet aren't included, unless the servant already knows such a spell. The servant can cast as many such spells as needed to fulfill a master's wish, at a rate of one per round, but cannot cast any such spell except on a target designated by the master; thus, a bound servant can't cast Polymorph on the master unless the master says "I wish I were a fly on the wall of the king's secret council chamber" or something else that could be fulfilled by a Polymorph ("I wish I were as strong as an ogre.")
A Wish Pact does not grant the servant any other special abilities, such as knowing where to find a magic flaming sword if the master wishes for one, or surviving the gaze of a basilisk if the servant is ordered to retrieve one. Thus, a bound servant may for example cast Contact Other Plane to locate such a magic sword, then Polymorph Self to become a dragon and fly the master to the location, and other spells as needed to help the master overcome obstacles.
If the servant dawdles or otherwise fails to comply with a wish, the normal affects of a Geas kick in. If, on the other hand, the master somehow breaks the agreement, the servant is no longer bound, even by any negotiated protection clause. Attempting to get more than you originally bargained for counts as breaking the agreement.
Some supernatural beings have an innate ability to cast a Wish Pact once a day, entering into a pact with a lesser being. Some use this as a bargaining chip to get something in return; others, particularly powerful demons, use it to spread chaos, eagerly offering wish pacts, usually for specific wishes negotiated in advance, in exchange for the "master's" soul when the pact has been fulfilled. Servants of Law may offer such pacts if the master agrees to a mission to aid their cause; such pacts usually stipulate that the wishes granted must in some way be linked to the mission.