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The nature of magic

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Quark

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Great Adventurer
Great Adventurer
For a high magic game, Pathfinder actually does a terrible job of defining how magic works, or even some basic principles of it. Taking a look through detect magic, I realised that spell is the only place where lingering auras of magic are actually mentioned, and it has loads of holes in it. There are even some things that I'd think would be fairly basic that aren't even touched on, anywhere. So, in this post, I'm going to rectify some of these problems.


Locations and sizes of magical auras:

When you cast a spell, you leave some residual magic behind. Detect magic inexplicably doesn't say where. It could be where it was cast, or where it took effect, and there is a significant difference between these two.

If you cast a fireball, there's a pretty big difference between where it takes effect and where you cast it. In part because the two are (probably) mutually exclusive, but also in shape. A fireball's a 20 ft. radius sphere, while most casters are sort of 5 ft. cube blobs (approximately).

It makes most sense to me that both the caster and the area of effect should have the magical auras. As a caster you have to actually manipulate the magic, and the effect of course is magic. The effect should leave a trace of approximately its size, and the caster would presumably leave a caster shaped blob behind. Maybe in a weird pattern to match the hand gestures and sound waves.


Wait, it's blurred?

While the shape of the aura is all very well for instantaneous spells, what happens if you have a more mobile effect? Take for example, a flaming sphere. Usually, you're going to be moving a flaming sphere around a lot for maximum utility. So, the question is, where's the aura lingering?

The obvious solution in this case is the ongoing spell has full aura since the magic's still all there and in effect. Once you've moved it the aura fades because the effect is no longer there. Consequentially, the flaming sphere must be leaving a magical trail behind everywhere it goes. This at least can be properly simulated with detect magic, once you've thought about it.

Returning to the thought that the caster also needs an aura, if you're concentrating on a spell and move, you must also leave a magic trail since you're still actively controlling the magic. From this, we can also reason that any spell that allows you to continuously alter aspects of a spell, such as changing the orders you're giving to someone via dominate person, must also stick around for some time afterwards, probably in the same way that the aura of the spell lingers on a dominated person. Importantly, this means that an ongoing spell that can be controlled can be dispelled by targeting either the caster or the effect.

One question this leaves is whether all spells, regardless of duration, will leave auras around the caster after the initial casting, even if you don't control it (such as mage armour on another creature). In this case, it's reasonable to assume that the aura doesn't linger around you for the entire duration of the spell, since you no longer need magic to maintain it or alter it; it's purely self-contained.

Finally, what happens when you cast an instantaneous spell and then move? Or equivalently, what happens to the aura of an expired spell that hangs around you when you move? While it would be nice for a tracking point of view to give an indication, since there's no more magic being used there's no reason for an aura to spread.


Residual stacks

For some reason, even if you have 100 casters all dumping a fireball in exactly the same spot at the same time, the aura only lingers for its normal duration. That is, 1d6 rounds. Frankly, this seems dumb, because that's an awful lot of magic being used there for very little residual. At the very least, you'd expect the aura to last somewhat proportionally to the number used.

Given how improved counterspell works and that you can identify different auras even a long time after they were left, it's probably reasonable to assume that auras of the same school can merge and interact in some way, while other types cannot.  That is, it's easier to detect two scorching ray castings than one acid arrow and one scorching ray.

It would seem reasonable to assume effects like this should stack in some way. If we keep our 100 fireball example, a reasonable approximation would be that the aura should last 100d6 rounds: roughly the same as a strong aura of 1d6 x 10 minutes. However, 100 fireballs detonating simultaneously would be much more dangerous than any normal equivalent (9th level spells don't even compare to that), so it's unlikely to be simple addition of aura to determine the duration.

Keeping it sort of arbitrary by using spell levels, a more realistic approach would be to add spell levels together in some way to determine duration rather than just their durations. Maybe it should work on a system similar to CR: You need 1.5 1st level spells to make a 2nd, 2 for a 3rd, 4 for a 4th, 8 for a 5th, 16 for a 6th, 32 for a 7th, &c.  That sort of exponential scale seems roughly right, since the actual time durations for higher level spells scale at approximately the same rate. 8 times as much magic for an aura that lasts 10 times longer; not perfect, but close enough. This makes extended caster battles leave as big a magic dump as you might expect for all the magic floating around.

Difficulties start appearing when you've got multiple schools. Since it seems logical that different auras can't merge, does it make sense if in the 100 fireball example, 5 people also cast summon monster in the same area that the additional conjuration aura lasts just as long? There's certainly an argument for it sticking around for a while longer than it should do, but it would probably be overwhelmed by the massive evocation aura from the fireballs.


How auras fade

All detect magic says about residual auras is that they fade. How helpful. There are two ways I can think of that might explain this behaviour: either magic decays like radioactive nuclei, or it just dissipates in a manner similar to coloured water in regular water.

The decay model works well in combination with the exponential model of the auras stacking; one basically explains the other in that it's sort of a half-life model. It allows auras to keep their shape much more with time, since there's no physical drift of the aura, and can explain how you can even detect ongoing effects (there's always going to be some decay because it's a half-life, and you just detect the decay).

On the other hand, this model doesn't deal so well with permanent effects so well, or longer lasting effects. If the effect lasts a long time, that's indicating it's not decaying, but my reasoning for how detect magic works in this model indicates you need some decay to detect it. The only way I can think of to allow this to work would be some sort of linear rather than exponential decay of a magical effect. Regardless, this only works for spells with finite duration. Permanent spells have to either be just really long lived or undetectable, which is its main flaw.

Dissipation has the same issue with permanent spells, but again, works in a similar manner to the decay model with its decay; it should be a half-life like decay. It has one advantage over the other model in that it can better explain how auras are left as the effect moves, but has the disadvantage of making sure auras don't keep their shape.

This section has been rather train of thought, but I'm thinking that the dissipation model works best. It's better to use the model where the auras are left behind as an effect moves than use the one where they keep their shapes better.

A consequence of this is that if you do have multiple strong auras in place that don't mix well, as it seems likely the auras of various schools of magic do, there'll be some artificially enhanced auras. If we take the example of 100 fireballs and 5 summon monsters all in the same place from above, the conjuration aura isn't able to properly disperse until the evocation aura is gone, so its duration will be lengthened despite its low intensity. It's possible it would be condensed by the force of the evocation aura attempting to push into its space as well.

If magic's just dispersing as a fading mechanism, even if it blurs and fades a lot, the magical aura will actually remain for much longer than detect magic indicates. It'll be quite weak, but in principle the auras should still be in slightly higher concentrations around the original location of a spell. If you were skilled enough, you should be able to piece together what spell was present based on what little remained of an aura, even once it had largely faded.


Learning things about spells

We already reasoned that when you cast a spell it leaves residual magic at both the location the effect took place and where you cast it. Since these both come from the same spell, it seems reasonable that if you have access to one, you can find out some things about the other. Sort of like a receiving an email, there's a way of tracing where it came from, although it's probably harder than just reading the "from" section. It seems reasonable that if you knew how to, you could also use that connection to learn everything there would be to know about a spell.

Similarly, since it makes sense for moving magical effects to leave auras behind, it also makes sense that you can tell some of the things that a spell has done from its residual aura. It would be easy to track where a flaming sphere has travelled for example. In what I hope is the first big assumption of this discussion, presumably you can also tell things about other parameters of a spell based on what the actual aura looks like. This could be things like what monster was summoned, what a suggestion was, what there was an illusion of, &c. Possibly even the caster level of the spell and DC. If you can tell where a mobile effect has been, it isn't a huge logical leap to assume that there are other obvious signs that give you more information about what a spell has done.

At last, increasing our reference material to beyond the detect magic spell, identifying the caster. Wizards need time to interpret a spell on a scroll or from another's spellbook; each wizard apparently has their own way of interpreting things so that they make sense. This means that even casting the same spell, each wizard likely has their own subtlety different way of casting. Basically, all wizards have a unique magical fingerprint. It's a fairly safe bet that the same thing holds for all magic users in the universe, not just wizards. Ergo, if you know what to look for, you can identify a caster based on the residual magic aura their spells leave.


What about magic items?

According to detect magic, you only get a residual aura when you destroy a magic item. This I'm going to call out as one of the bigger holes in the spell; it's got an aura, so it's going to leave some trace of that behind when it moves.

These should behave in more or less the same way as other magical effects, save that they are entirely self-contained: there is no place where the effect originated. They can also be considered casters, since many items cause new magical effects.

Spell completion items like scrolls have the interesting characteristic that since they're spells almost completed by someone else, requiring you to just perform the last few actions, the magical fingerprint will be a combination of both the item's creator's and your own. This doesn't hold for spell trigger and command word items, that are effectively entirely cast by someone else; these have a magical fingerprint that is entirely the creator's.

It's debatable whether such items might have an aura relating to their form anyway. Since there are feats for making a variety of different items, it's not unreasonable to assume that there's a certain, unique way for each item type to have its magic put into it, in which case you can tell if an item or effect comes from a scroll, potion, magic weapon, &c..

Since magic items tend not to have spell levels, but instead have caster levels, the method I proposed for stacking auras isn't going to work here, but you can get something functionally identical if you convert caster level to spell level: just divide the caster level by 2 (minimum 1) and you get its equivalent spell level.

Appropriately, this means that what is commonly known as the Christmas Tree effect ends up being suitably blinding, even quite some time after the tree has left.

Summary


  • Spells leave auras in distinct halves: one at the casting site and one at the effect site
  • Ongoing magical effects leave auras behind as they move
  • Spells that require continuous control are detectable on both the caster and effect, and as such can be dispelled by targeting either the caster or the effect
  • Multiple auras of the same school stack to create longer lasting residual auras
  • Different school auras don't mix, but can cause each other to last longer
  • You can learn things about a spell from the aura where it took effect or where it was cast
  • Details of a spell can be learnt by examining their auras
  • Each caster has a unique magical fingerprint
  • Magic items function like spells for the purposes of auras
  • You can identify the type of magic item by its aura



Expanded Rules

To finish off, because I can't resist a good bit of homebrew, here are some highly experimental rules to properly round off this post.

Aura strengths and durations

This is intended to expand upon the table in detect magic. These will be necessary to properly account for increased aura strengths, and also allow for the fact that you can get higher than 9th level spells through metamagic rods and gems. The ordering is in the format strength: Functioning spell (spell level). Magic item (caster level). Duration of lingering aura.

Weak: 3rd level or lower spells. Caster level 5th or lower. Aura lasts 1d6 rounds.
Moderate: 4th to 6th level spells. Caster level 6th to 11th. Aura lasts 1d6 minutes.
Strong: 7th to 9th level spells. Caster level 12th to 17th. Aura lasts for 1d6 x 10 minutes
Overwhelming: 10th and 11th level spells. Caster level 18th to 21st. Aura lasts for 1d6 days
Crushing: 12th and 13th level spells. Caster level 22nd to 25th. Aura lasts for 1d6 weeks
Deific: 14th + level spells. Caster level 26th +. Aura lasts for 1d6 months

When multiple lingering auras are present in the same square, all auras linger for as long as the strongest aura. When multiple auras of the same school are in the same square (so long as they don't result from the same casting of a spell), the strength of the aura increases. The aura strength is determined by adding the spell levels of the individual auras together in the same way that CRs add, treating the spell's level as its CR. For the purposes of this, magic items are considered magical effects with a spell level equal to half their caster level (minimum 1).

Knowledge(Arcana):

You can use your knowledge of magic to better learn about the history of a spell.

Learn how long ago a spell was cast  This is a DC 20 check that takes a full-round action to perform, or a standard action if you do not need to maintain concentration on detect magic. This is only accurate to the units of measurement used to determine the lifetime of the aura (i.e. a weak aura gives a time in rounds, a moderate aura gives a time in minutes &c.)

Increase the effective strength of an aura by one step  This is a DC 30 check, plus 10 for each additional strength category advanced and takes a minute per strength category advanced. This duration is halved if you do not need to concentrate on detect magic. This does not actually increase the strength of an aura, but it allows you to detect more than you might otherwise be able to.

Physically track a magical effect by residual auras  You can tell where the bright spots in a magical aura are to tell which direction it travelled. This check is DC 20 and takes a full round action to perform or a standard action if you do not need to concentrate on detect magic. You can travel up to half the range of your means of magical detection while attempting to track an effect with no penalty. You can move faster up to the range of your magic detection by increasing the DC by 5.

Identify a magical fingerprint  You can identify a caster's unique magical fingerprint in a residual aura with a check of DC 20 + the spell level of the effect. If the spell originated from a spell completion item, the DC increases by 5, but you identify both the item's creator and the creature that activated the item.

Determine if an aura is from an effect or the caster This is a DC 15 check, performed as part of the check to identify the school of an aura.

Determine if an aura comes from an item or caster This is a DC 20 check. If successful, you identify the type of item the aura comes from (such as from a scroll, potion, wondrous item, magic weapon &c.)

Spellcraft

You can identify various properties of a spell based on its residual aura.

Determine caster level This is a DC 20 + spell level check that is performed as part of the check to identify the spell.

Identify spell parameters You can identify what choices were made or being made in the casting of a spell with a DC 25 + spell level check. This includes the type of monster summoned with summon monster or the words to a suggestion. If the spell has a duration of concentration, can be dismissed or allows additional input once cast (like dominate person, the DC is instead 35 + spell level.

Track aura halves If you can see an aura where a spell was cast, you can determine where the effect initially took place and vice versa. You can determine the current location of the caster from an ongoing effect for any spell with a duration of concentration, can be dismissed or allows additional input once cast (like dominate person) and vice versa. Both uses require a DC 20 + spell level check, subject to modifiers outlined below:

Other half is out of range of detection method: +5*
Other half is more than 1 mile away: +5
Other half is on another plane: +10

*This stacks with other penalties that may apply (because sometimes you have very long range detection mechanisms)


Identify Summons
School Divination; Level Sorcerer/Wizard 5, Psychic 5, Summoner 5, Inquisitor 4
Casting Time 1 standard action
Components V, S, F (residual aura of a spell of the calling or summoning subschools)
Range Personal
Target creatures summoned with a spell of the calling or summoning subschool
Duration instantaneous
Saving throw Will negates, see text; Spell resistance no

This spell draws upon residual aura of a spell of the calling or summoning subschools and gathers the information required to resummon the same creature(s) summoned using that spell. The creature(s) is entitled to a will save to negate this effect.

Replay Illusion
School Illusion [figment]; Level Sorcerer/Wizard 3, Psychic 3, Occultist 3, Mesmerist 3, Bard 3, Inquisitor 2
Casting Time 1 standard action
Components V, S, F (residual aura of a spell of the figment or glamer subschools)
Range medium (100 ft. + 10 ft./level)
Duration concentration
Saving throw none; Spell resistance no

This spell utilises the residual magic of a figment or glamer to replicate its effects, but the effects are clearly illusions. The effect plays chronologically in real time from the earliest point within range, but can be fast forwarded or rewound at up to 10x speed and stopped at will. The effect only extends as far as this spell, but you can move to change how much is visible.

Lesser Strengthen Auras
School Universal; Level Everything but alchemist 1
Casting Time 1 standard action
Components V, S
Range 30 ft. radius burst, centred on yourself
Target residual magic auras
Duration 1 minute/level
Saving throw no; Spell resistance no

This spell temporarily focuses residual magical auras, increasing their strength by one step, to a maximum of strong.

Strengthen Auras
School Universal; Level Everything but alchemist 4

This functions as lesser strengthen auras, but it instead increases the strength of magical auras by two steps, to a maximum of overwhelming.

Greater Strengthen Auras
School Universal; Level Everything that gets 7th level spells 7

This functions as lesser strengthen auras, but it instead increases the strength of magical auras by three steps, to a maximum of crushing.

Lesser Aura Disperse
School Universal; Level Everything but alchemist 1
Casting Time 1 standard action
Components V, S
Range 30 ft. radius burst, centred on yourself
Target residual magic auras
Duration instantaneous
Saving throw no; Spell resistance no

This spell disperses magical auras, making them harder to detect.  All spells in the area are treated as if 1 minute had passed. This spell does not leave an aura.

Aura Disperse
School Universal; Level Everything but alchemist 4

This functions as lesser aura disperse, but it instead treats all auras as if 10 minutes had passed.


Greater Aura Disperse
School Universal; Level Everything that gets 7th level spells 7

This functions as lesser aura disperse, but it instead treats all auras as if a day had passed.


Anonymous Spell (metamagic)
Your magic lacks any distinctive characteristics, making it challenging to identify
Prerequisites: Knowledge(arcana) 5 ranks, Spellcraft 5 ranks
Benefit: The DC to identify the caster of a anonymous spell by their magical fingerprint increases by 5. A spell modified by this metamagic feat is only identified as being altered by this feat if the DC to identify the spell with spellcraft is beaten by 5 or more.
Level Increase: +1 (An anonymous spell uses a spell slot 1 level higher than the spell's actual level)


Incriminating Spell (metamagic)
You alter your casting style to replicate another caster's magical fingerprint
Prerequisites: Anonymous Spell
Benefit: An incriminating spell has a magical fingerprint that looks like another caster's. A spell modified by this metamagic feat is only identified as being altered by this feat if the DC to identify the spell with spellcraft is beaten by 10 or more. A creature that succeeds on the knowledge(arcana) check to identify a magical fingerprint by 10 or more learns the true identity of the caster.
Level Increase: +2 (An anonymous spell uses a spell slot 2 levels higher than the spell's actual level)
Special: You must be familiar with the magical fingerprint of the caster who you wish to imitate.


Trackless Spell (metamagic)
By sending all of the magic to a location in one go, you can make it substantially harder to track either half of the magical auras.
Benefit: A trackless spell obscures the connection between the auras of the casting location and the origin of the effect. The DC to locate the other half of a trackless spell's aura increases by 10.
Level Increase:  +1 (A trackless spell uses a spell slot 1 level higher than the spell's actual level)
Special: Spells that can be dismissed, can accept additional input once cast or with a duration of concentration cannot be modified by this metamagic.


Quick Fade Spell (metamagic)
You can adjust your magic so that any residual aura disperses more quickly.
Benefit: A Quick Fade spell is treated as one strength category lower for the purposes of its lingering aura. If this would reduce an aura's strength to below weak, it instead leaves no aura. This metamagic feat does not count towards spell level for the purposes of determining the strength of a magical aura.
Level Increase:  +1 (A trackless spell uses a spell slot 1 level higher than the spell's actual level)
Special: This metamagic feat can be applied to a spell multiple times. Its effects stack.


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