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In Pathfinder and 3.5, the Antimagic Field spell has a duration of ten minutes per level, and only extends 10 feet past the caster. However, I frequently see references to antimagic fields that are cast on prisons, buildings, or other areas.

It seems like this would require a vastly extended duration and range for this to be practical, not to mention the ability to cast it on an object or keep it from moving with the caster.

How is this justified? Is it just a matter of "us" vs. "them", requiring the PCs to follow the rules while allowing the game world to vary as needed? I don't like the sound of this, as I don't want to forbid one of my players from doing something that others have done simply because there aren't really rules for it. Without a clear justification, it's a nice idea, but when my wizard asks to protect the PC's stronghold with AMF, I want to be able to give him the result of his research.

To clarify, I'm not looking to be convinced that it's possible; as the GM, I decide what is possible. What I'm looking for is how it comes to be. I don't want to arbitrarily state that it exists; I want the world to make sense. That is, I want every character-created spell or effect to have an actual cost or requirement associated with it, so that I can reasonably determine who can make use of it.

Examples: This question and some of its answers
This Order Of The Stick comic strip, sort of

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3 Answers 3

up vote 18 down vote accepted

Not Every Option Is Explicit

The entire point of the ruleset presented in the Dungeons and Dragons roleplaying system is that it does not cover every conceivable circumstance. Wizards create custom spells, strange creatures never heard of before can exist in ancient forgotten forests, feats and classes and powers that are not in the rulebooks should abound. The DM is given the role of gatekeeper for this sort of stuff, as they are supposed to be impartial about content in a way that players aren't.

Any GM who tries to stick strictly to the existing content, and does not allow things like 'using skills in an unlisted way' or new spells or whatever, to the limit of balance and verisimilitude, is missing out on a large portion of the game.

Explicit Options

  1. Permanency has a short list of allowed spells, but adds that the DM can add others. Antimagic Field is an obvious candidate.
  2. Flux Slime is a natural planar substance that creates an anti-magic field.
  3. Dead Magic Zones (DMZs) - They occur naturally or 'around the sites of magical cataclysms' (in faerun). They operate as an AMF in most ways, except in faerun (shadow weave magic still works).
  4. Resetting Traps - A Self-Resetting Antimagic Field Trap is CR 7 and costs 33,000gp, creating a permanent area of antimagic.
  5. Energy Transformation Field - A ETF keyed to Antimagic Field with an automaton activating an infinite use magic item (such as three *immovable rod*s) inside a space inside the field continuously, casts the spell on a continuous basis every time it goes down.
  6. Stronghold Builder's Guidebook (updated 3.0 material) has Sigils of Antimagic, which add Antimagic properties to a single 'stronghold space'. It costs 66000gp.

there are more Wizard Tricks that can replicate this sort of thing.

Simple Houserules

Whether you're allowing expanded, permanent, static antimagic fields as a use of the permanency spell, as magic devices (death star force field generator style), or as a custom spell, it should cost XP to cast.

XP is far superior as a costing mechanism for permanent effects than GP as it is a straightforward measure of power instead of the weird in-between of GP, and there's no real way to arbitrarily get more of it. Without cheesy methods of regaining XP (Thought Bottle, Negative Level/Restoration Dance, Dweomerkeeper) it is a steady, measurable cost for doing business.

Assuming it affects more than just a 10' radius area, 1000 XP is a fair price for a permanent antimagic field. One 10' cube per level per cast seems fair.

Simple Implementation

Antimagic Zone

Abjuration

Level: Clr 9, Magic 7, Protection 7, Sor/Wiz 7

Components: V, S, M/DF, XP

Casting Time: 10 Minute Ritual

Range: Long (400' + 40'/Level)

Area: One 10' Cube/level

Duration: Permanent

Saving Throw: None

Spell Resistance: See text

An invisible barrier surrounds a space you designate. The space within this barrier is impervious to most magical effects, including spells, spell-like abilities, and supernatural abilities. Likewise, it prevents the functioning of any magic items or spells within its confines.

An antimagic zone suppresses any spell or magical effect used within, brought into, or cast into the area, but does not dispel it. Time spent within an antimagic zone counts against the suppressed spell’s duration.

Summoned creatures of any type and incorporeal undead wink out if they enter an antimagic zone. They reappear in the same spot once the field goes away. Time spent winked out counts normally against the duration of the conjuration that is maintaining the creature. If you cast antimagic zone in an area occupied by a summoned creature that has spell resistance, you must make a caster level check (1d20 + caster level) against the creature’s spell resistance to make it wink out. (The effects of instantaneous conjurations are not affected by an antimagic field because the conjuration itself is no longer in effect, only its result.)

A normal creature can enter the area, as can normal missiles. Furthermore, while a magic sword does not function magically within the area, it is still a sword (and a masterwork sword at that). The spell has no effect on golems and other constructs that are imbued with magic during their creation process and are thereafter self-supporting (unless they have been summoned, in which case they are treated like any other summoned creatures). Elementals, corporeal undead, and outsiders are likewise unaffected unless summoned. These creatures’ spell-like or supernatural abilities, however, may be temporarily nullified by the field. Dispel magic does not remove the field, though Mordenkainen's Disjunction might.

Two or more antimagic fields/zones sharing any of the same space have no effect on each other. Certain spells, such as wall of force, prismatic sphere, and prismatic wall, remain unaffected by antimagic zone (see the individual spell descriptions). Artifacts and deities are unaffected by mortal magic such as this.

Should a creature be larger than the area enclosed by the barrier, any part of it that lies outside the barrier is unaffected by the field.

Arcane Material Component: A pinch of powdered iron or iron filings.

XP Cost: 1000 XP.

How is this justified?

Magic is assumed to be, in many ways, a Field. A localized, saturated energy that can be called upon to perform magical effects, power magical devices, and empower magical creatures.

Without that magical field (such as in the area of an antimagic field or a dead magic zone), spells do not work, dragons cannot breathe flame, magical items do not function.

The Antimagic Field spell specifically destroys all magical energy inside it's radius of effect. A permanent version of that spell is an effect that does that on a permanent basis.

That's the in-game justification of that. A magical effect that in some way removes all magical energy from an area (by blocking it off, or using it up, or whatever).

There is no standardized system for costs and requirements for custom spells and effects

The DM has to set that. Rule of thumb, eyeballing, comparative analysis, dark powers of Chaos, call it what you will, the basic mechanical art of encounter design looking at things and evaluating them for mechanical balance, and is a key skill for any GM. Applying that to spells is a simple matter of reading through similar effects and estimating what you need to do to create a spell, feat, ability, magic item, or naturally occurring (Na) effect to mechanically iterate the effect or ability you are trying to create in the world.

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1  
+1 for the second paragraph! –  Squera May 14 at 8:08
    
@EnvisionAndDevelop If I could provide an actual reference, I would post it as an answer. I was (maybe excessively pedantically) taking exception to your comment "they occur naturally, (so) you can't create them" -- that's a non sequitor, which is all I was trying to say. –  Kromey May 14 at 15:00
    
Please limit the back and forth in comments - amend the question and answer with any useful clarification herein and flag for removal. –  mxyzplk May 14 at 23:53
    
Excellent answer. I'd add one minor point. The spells that are listed in the books for spellcasters are designed to be a list of what can be cast on the fly and on the road, not a comprehensive list. A spell cast over an entire prison, for example, would likely be something that is prepared and cast over a period of days, weeks, or even months, with requisite increases in materials and costs. That would, to me as a GM, be sufficient of itself to justify the much larger area of effect and allow for the addition of permanency to be implemented. –  BBlake May 15 at 18:58
    
@BBlake If that were in an answer, I would upvote it. Interestingly enough, I think that is a pretty satisfying answer, although it's pretty simple. –  EnvisionAndDevelop May 16 at 3:01

There are no RAW rules for making permanent Anti-magic Fields.

In 3.0, there was a rule in the Stronghold Builder's Guide which let you place an entire building under the effects of an Anti-magic Field (Sigils of Antimagic, page 83), but that's not natively allowed in either 3.5 or PF. Most of the time, permanent AMFs are made by GM fiat. The idea of a jail that prevents spellcasting is a compelling one, and all a GM has to do to make it is say that it exists. It's pretty rare for a player to want to create a structure like that, so usually that kind of thing is also handled by GM fiat. However, there are a few ways that you can houserule a permanent AMF into existence.

  1. Permanency: The spell Permanency doesn't natively allow AMF. However, it wouldn't be that hard to think up a reasonable price for it, and add it to the list. Animate Objects and Symbol of Fear are both 6th level spells that are on the Permanency list, and they cost 15000 gp (or 3000 XP in 3.5), so that's probably a good price. I don't think it would be unreasonable to allow a player to cast a permanent Anti-magic field using Permanency, with a minimum caster level of 14 and a cost of 15000 gp (or 3000 XP in 3.5).

  2. Magic Item: You could theoretically make an item that creates a constant AMF. Creating a magical tile that can be incorporated into a building would cost 396000 GP, based on my calculations. That would be 6 (spell level) * 11 (caster level) * 2000 (continuous) * 1.5 (duration modifier) * 2 (doesn't take a slot). This isn't very cost-effective, since that only gives you a 10 foot radius of antimagic. You could probably drop that price quite a bit, especially if it is only effective when incorporated into a building, and can't be carried around. If I were pricing such an item in one of my games, I'd charge 33000 gp (6*11*2000/4), mainly because of how limiting an AMF can be. That would put the item within the purchasing power of a large magic item shop or higher level players, without being crazily expensive.

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It is justified the same way that magical spells are imbued into weapons, armor and equipment. When you craft a magical item the cost of the item varies based off of the Caster Level, the Duration of the effect, Charges Per Day, and so on. In the game world these sorts of scenarios are affordable due to the time and years of planning.

You as the PC could not have obtained enough gold to make such a magical item in such a short point in time. Where as say the prison has been around for over 100 years, there is plenty of time for casters to have worked towards, and contributed to, such a costly project.

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