What ways are there to magically enforce an agreement?

Question

Assume a PC (or the whole party) wants to make a deal with another PC or NPC. What magical ways are there to make this "contract" binding and enforceable, or at least somewhat so?

Examples where this might be useful:

• An undead mage will surrender (with his allies) and give the party the MacGuffin, but only if they agree to kill him and then reincarnate/raise him (note: this kill/raise trick works in my campaign) and set his newly-alive self free.

• The party wishes to let the enemy bandits live and leave if they drop their weapons and give back the loot. And then only take into custody some of the bandits who actually murdered people.

Etc.

To suit these kinds of situations it has to be something doable on the spot and without mortal authorities backing either side of the agreement — hence, magical.

Answer 1

Get an Actual Magical Contract

The contract of Nepthas (Complete Arcane 148) (1,400 gp; 0 lbs.) leaves

[t]he details of the contract... blank, and the user can fill it in with any instructions, agreement, or conditions he cares to create. When the contract is signed, though, the item’s true power is revealed to both parties, and any signatory who breaks the contract is subject to a curse that strikes it blinded, deafened, and mute (no saving throw, though spell resistance applies). The curse of a contract of Nepthas can be removed only by means of remove curse cast by an 8th-level spellcaster or a break enchantment spell (DC 25).

Because a contract typically involves two parties agreeing on a set of conditions, nonspecific terms might allow a clever signatory to escape them without suffering the contract's curse....

Contracts [n.b.--with a grain of salt--in general as the word's not in italics; who knew such vital information was in a splatbook's magic item description?] signed by creatures under the influence of charm or compulsion effects are null and void.

That more than one creature can be a contract's signatory is implied by the last paragraph, but this isn't stated, so ask the DM. Further, if the DM says that's possible, the DM must also determine what happens after all the bandits sign then one bandit breaks the contract. (However, the number of signatories doesn't appear to change the contract's power--the contract's effect is never discharged--, rendering a lone, rebellious, contract-breaking bandit affected by the curse yet leaving unaffected the other bandits unless they later individually break the same contract. Again, ask the DM.)

Having a contract of Nepthas means planning ahead, but the item's priced reasonably and doesn't require a caster to complete. A wealthy, lawful kingdom might equip trustworthy elite soldiers with contracts of Nepthas, urging them to impress defeated enemies into the kingdom's ranks.

The only real disappointment is the ease of removing the contract's curse. Although the contract is created at caster level 11, the contract's description appears to be written so an adept (DMG 107-8) capable of casting the appropriate spells at all can use the spells remove curse [abjur] (PH 270-1) or break enchantment [abjur] (PH 207) to relieve the consequences of breaking the contract. A sufficiently high-level adept can sometimes be found in a large town but more often a small city (DMG 139). The curse has no saving throw, though, and being blind, deaf, and mute is pretty severe, so first getting the adept to agree then breaking the contract is a good idea.

Because of the caster level at which the contract's created, a reasonable house rule to remove the curse sets the minimum caster level to 11 for the spell remove curse and the caster level check DC to 22 for the spell break enchantment. This permits scaling up the contract if a more powerful contract is desired.

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Background
In trying to determine the identity of Nepthas, I learned that in Advanced Dungeons and Dragons, 2nd Edition the contracts of Nepthas (originally from Tome of Magic (1991) but reprinted on page 329 of Encyclopedia Magica, Volume 1 (1999)) required signatures from both parties, allowed any reasonable number of signatories, required a deadline to be set, released from the contract creatures that die, prohibited forged signatures, and used the same example ("Slay a dragon in the Northern Hills") as Complete Arcane uses over a decade later.

And I still don't know who (or what) Nepthas was.

Answer 2

One (or both) party(s) should cast the Call Kolyarut spell (Planar Handbook) to have Kolyaruts serve as a witness to the deal.

A Kolyarut (thanks Ruut) has as its obsession:

Kolyaruts mete out punishment to those who break bargains and oaths.

The spell requires the statement of a task. In this case, the task is "witness the following contract, agreed to by both parties." This is a very short task, but at least means that the inevitables will be watching for default in subsequent months. One or both parties may specify resources in escrow (and give those resources to the Kolyarut) to be paid or withheld in default for additional enforcement mechanisms.

For the MacGuffin exchange, it's very straightforward.

Caster¹ begins: I, as caster [without prejudice to my involvement in entry into the subsequent contract], call upon this Kolyarut to choose if they are willing to witness the following contract, to serve as an escrow agent, and, if choosing to serve as an escrow agent, give the parties involved a name or designation which may serve to complete the transaction waiting in escrow.

The BBEG opens: I, the big bad evil guy, place the MacGuffin in escrow with the Kolyarut designated, (name). The MacGuffin is to be released to the individuals known as (party name) after they have successfully restored me to full life in a period of not more than one sidereal month to this date.

Party responds: We, the party of the heroes of good, acknowledge that the MacGuffin is in escrow for our services. Our services entail destruction of the undead form of the BBEG and subsequent resurrection as a living creature. In return for our activity, we will summon the Kolyarut known as X upon demonstrated completion to complete the exchange.

Then, when the party complies with the exact wording of the contract, they summon the inevitable, get the MacGuffin, and everyone is relatively happy.

Specifically, with the ability to summon the attentions of the inevitables, the individuals in question are engaging in triple-entry bookkeeping, much like a normal real-world real estate deal with an escrow broker who can ensure successful completion (or rolling back) of a deal.

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¹ The neutral caster may be a member of one or either party, but is acting in a separate capacity.

Answer 3

The classic D&D spell for enforcing agreements or compelling behavior is geas:

A [..] geas places a magical command on a creature to carry out some service or to refrain from some action or course of activity, as desired by you. The geased creature must follow the given instructions until the geas is completed, no matter how long it takes.

^{Originally from Celtic mythology, where a geas, or geis, was a vow taken or a limitation placed upon a hero).}

I can easily see a reciprocal geas being cast on both parties, both committing to the agreement. Note, though, that despite the very narrative feel of this spell, it's still a D&D spell, and can be broken with remove curse and similar remedies.

Answer 4

Homebrew Solution:

Geas/Quest, Mass

Enchantment (Compulsion) [Language-Dependent, Mind-Affecting]

Level: Clr 9, Sor/Wiz 9
Components: V
Casting Time: 10 minutes
Range: Close (25 ft. + 5 ft./2 levels)
Target: One or more creatures, no two of which can be more than 30 ft. apart
Duration: One day/level or until discharged
Saving Throw: None
Spell Resistance: Yes

This spell functions similarly to Geas, except as noted above.

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Mark of Justice, Mass

Necromancy

Level: Clr 8
Components: V, S, DF
Casting Time: 10 minutes
Range: Close (25 ft. + 5 ft./2 levels)
Target: One or more creatures, no two of which can be more than 30 ft. apart
Duration: Permanent
Saving Throw: None
Spell Resistance: Yes

This spell functions similarly to Mark of Justice, except as noted above.

Rationale behind the spell research

Geas is a 6th level spell. Heal is a 6th level spell. Heal, Mass is a 9th level spell. So a mass version of Geas should be 9th level.

Mark of Justice is a 4th level paladin spell. Cure Critical Wounds is a 4th level spell for clerics. Cure Critical Wounds, Mass is an 8th level spell for clerics. So a mass version of Mark of Justice should be 8th level.

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SRD Solution:

Inevitable Intervention

Inevitables are constructs whose sole aim is to enforce the natural laws of the universe.

Each type of inevitable is designed to find and punish a particular kind of transgression, hunting down a person or group that has violated a fundamental principle. When an inevitable is created, it receives its first mission, then finds the transgressors and metes out appropriate punishment. The sentence is usually death, although some inevitables insist on compensation to the wronged party instead, using geas and mark of justice to ensure compliance. From its first step, an inevitable focuses totally on its target. It continues its efforts no matter how cold the trail or hopeless the task. Inevitables are single-minded in pursuit of their quarry, but they are under orders to leave innocents alone. Accomplices to their prey are fair game, however, which sometimes creates conflicts within their programming.

Inevitables gladly sacrifice themselves to complete a mission, but they aren't suicidal. Faced with impending defeat, they are likely to withdraw and seek a way to even the odds. They are determined but patient foes. They ally with others if that helps accomplish their mission, but they have a hard time keeping allies for long.

Inevitables tend to stick out in a crowd while they're in observation mode, but they seem oblivious to the attention. Their forms vary, but all inevitables are gold-and-silver clockwork creatures, with gears and pistons where muscles would be on flesh-and-blood creatures. Their eyes glow with a golden radiance.

Note that unlike most constructs, inevitables have an Intelligence score and can think, learn, and remember.

List of Inevitables

• Kolyarut
• Marut
• Zelekhut

Rationale Behind Inevitable Intervention

Your contractual bindings is an application of law - something that the entire Plane of Mechanus is made of - law. As DM you could easily insist that every contract and agreement is supervised by the Inevitables, and they could be your arms of enforcement - since that is what they do.

Source: http://www.dandwiki.com/wiki/SRD:Inevitable

To quote a snippet from a popular hip/hop album from the 1990's: "He who breaks the law, shall be punished - back to the House of Pain."