How can an adventurer prevent trivial detection via Detect Magic and Arcane Sight?

Question

Most 3.5 adventurers, past the first few levels of their careers, carry a plethora of magical items and effects; this has been referred to as the "Christmas Tree Effect." This is fine if one's goal is to impress the locals with a display of magical badassery, but sometimes one might wish to, say, disguise oneself as a harmless peasant, or sneak past some guards.

Unfortunately for our would-be-stealthy adventurer, there are trivially-available ways of detecting magic:

• Detect Magic is a level 0 spell, available to pretty much everyone with any spellcasting ability, and easily usable from a wand by anyone with even modest investment in Use Magic Device. It detects "magic auras," which are given off by all magic items, all active spells, and things which have had spells cast on them recently.
• Arcane Sight is a level 3 sorc/wiz spell which, in addition to detecting all of the above without active concentration, also detects any creature capable of using spells or spell-like abilities, and can explicitly be made permanent.

This means that, however well our sneaking spellcaster might protect themselves from mundane modes of detection, they can expect to be caught by any sentry who has bothered with even minimal arcane preparation. In the world of 3.5, we should expect these sorts of measures from any security-minded and reasonably competent individual.

The only countermeasure I am aware of is the Magic Aura spell. Unfortunately, this spell only masks auras from magic items, and not from any of the other sources mentioned above. This will not, in most cases, be sufficient; adventurers who can't cast any spells, can't use any spell-like abilities, aren't under the effect of any spells, and haven't been affected by any spells lately are few and far between in the world of 3.5.

Is there any realistic way for an adventurer to protect themselves from this sort of easy detection? If so, how should they go about it?

Answer 1

You can't effectively hide active spells in 3.5 (in Pathfinder there's a spell for that, but not 3.5). If you wanna sneak, you gotta forgo active spells. You can hide magic items and your spellcasting abilities (along with other things like alignment and being alive), though. You need two spells:

• Nystul's Magic Aura, which lets you hide magic items
• Mind Blank, which lets you personally hide from all detections

Unfortunately, these spells themselves have auras, and thus can be detected by detect magic et al., curtailing their use in areas where an extra magical aura would be noticed.

The lower level spell Nondetection works similarly, except that it allows a CL check, and thus is extremely unreliable, as is Misdirection when not chain cast, as it allows a Will Save.

Generally, when sneaking into a place, you have a couple options.

First of all, if the opposition has no magic auras at all then they don't have detect magic spells, so you don't need to hide your auras.

If the only magic they have is divinatory then things are a little tricky. If they're using detect magic, they're still gonna need to study an area with you in it for 2 rounds to be able to detect your presence (1 round reveals only that there is magic, which is no longer enough to arouse suspicion, while 2 reveals the number of magic auras, which could trigger an alert when there's an extra). If they're using Arcane Sight then they need line of sight to get anything. As long as you're hidden, they won't be able to pinpoint much of anything about you.

If, like typical 3.5 opponents, they and their base and chock full of magic auras, you can pop some Nystul's Magic Aura spells and bring some potions, scrolls, or castings of Misdirection or Nondetection to hide yourself. Alternatively, you could splurge on a scroll of Mind Blank (if you're high enough level to cast this, then you should definitely just use this all the time), or use some of the other anti-divinatory effects. The extra magic auras shouldn't be of note as long as there aren't too many. Plus you can break enchantment or dispel magic preexisting auras in the area you are hiding to keep the number the same.

As a note, some people play 3.5 with the auras of active spells just modifying the auras of the thing they are on, if they are on a thing. This lets Nystul's Magic Aura et al. work to effectively hide their subjects. If you are playing in such a game, the following spells are significant:

• Nystul's Magic Aura
• Misdirection
• Nondetection
• Mindblank
• False Vision
• Detect Scrying
• Antimagic Field

Also of note: enemies that lack a parade of magic auras all over the place probably don't need to be sneakily handled. Just kick in the door and clean house, unless you've reason to suspect they're a spellcaster or something. Also, Greater Teleport removes most non-reconassaince reasons for sneaking anywhere not under a forbiddance (or, less commonly, a dimensional lock or hallow/unhallow(dimensional anchor)), since you could just pop in wherever you intended to go.

For the times you just want people to think you're a harmless peasant, Bluff checks can handle that even without magic, and with Glibness you can safely inform them that those magical auras they are noticing are just a trick of the light and also they never noticed them and you never talked and these are not the droids they are looking for.

So the situations you'll want to be actually using anti-divinatory powers as a major approach in are pretty few and far between, though they are pretty long-lasting and certainly worth lower level slots as you level up.

Answer 2

Nondetection (good!)

The 3rd level spell Nondetection can be used to ward an item, or a creature and all the gear it is carrying, against divinations:

The warded creature or object becomes difficult to detect by divination spells such as clairaudience/clairvoyance, locate object, and detect spells. Nondetection also prevents location by such magic items as crystal balls. If a divination is attempted against the warded creature or item, the caster of the divination must succeed on a caster level check (1d20 + caster level) against a DC of 11 + the caster level of the spellcaster who cast nondetection. If you cast nondetection on yourself or on an item currently in your possession, the DC is 15 + your caster level.

If cast on a creature, nondetection wards the creature’s gear as well as the creature itself.

The list of spells mentioned in the spell description is not exclusive; the subject is warded against "divination spells" generally, such as the ones mentioned. Any Divination spell effect attempted against the warded target requires a caster level check to succeed, otherwise it detects/reveals nothing. The Detect Magic and Arcane Sight spells are both divinations and thus warded against by the spell.

The warding effect affects a character, their gear, and any active spell effects on them as a single package; anyone attempting a divination against character would make a single caster level check to overcome the single Nondetection spell, and if they succeed they'd be able to sense any of the relevant auras on the character or their equipment.

Nondetection must also extend to spell effects currently on the subject itself, because Nondetection is such a spell effect; it wouldn't really do an effective job of hiding you from magical detection if the effect itself was trivial to magically detect. Additionally, the wording of the detect spells states:

... If the items or creatures bearing the auras are in line of sight...

Which suggests that the aura of an active spell effect is quite tangibly tied to the character/item it targets, and the warding of Nondetection should apply.

Since it can be overcome by a caster level check it's not as reliable a ward against detection as Magic Aura is for items, but it does give you a good chance of evading such divinations. When used by a caster on themselves it's more effective, increasing the caster level check DC from 11 + Nondetection CL to 15+, which would give the sneaky caster roughly 30% odds of being detected by an enemy caster of similar level (and very good odds against items casting Detect spells, since they will typically have very low caster levels).

It's available in item form as the Amulet of Proof Against Detection and Location.

Misdirection (okay)

Another spell useful for evading detection based on your auras is Misdirection, a level 2 spell which disguises your aura by making you appear to be something else; however, it is overcome by a simple Will save by the detector, so does not scale in effectiveness with your caster level so much. But you can also use Misdirection to pretend to have the magical auras of something else, so it has a little more versatility of intent.

Mind Blank (not without anti-RAI lawyering)

The Mind Blank spell could be considered to protect you from such detection spells if you read it a certain way, owing to the highlighted portion:

The subject is protected from all devices and spells that detect, influence, or read emotions or thoughts. This spell protects against all mind-affecting spells and effects as well as information gathering by divination spells or effects. Mind blank even foils limited wish, miracle, and wish spells when they are used in such a way as to affect the subject’s mind or to gain information about it. In the case of scrying that scans an area the creature is in, such as arcane eye, the spell works but the creature simply isn’t detected. Scrying attempts that are targeted specifically at the subject do not work at all.

If you take that as a separate statement it suggests that no divinations will work against the subject of the spell. RAW, there's quite the argument to be had. The intention, however, is fairly clearly that the protections of Mind Blank apply only to effects which would read or affect your mind, and to remote scrying attempts. The closest I could find to official statements on that are reposts of posts from elderly forums:

Mind blank stops things that effect the mind, and scrying (divinations that work remotely without line of effect to the subject). Mind blank does not stop direct observation with a divination spell (unless of read the mind).

and

Q: You recently answered a question regarding the Mind Blank spell vs. the effects of True Strike, stating that Mind Blank does not affect True Strike. Many people resting on both sides of the issue we're hoping you could offer an explanation as to why it does not.

A: True strike doesn't reveal anything about a particular creature, so mind blank has no effect on the spell.

Q: In addition, there have been questions raised concerning how Mind Blank functions in conjunction with other spells. For example, lets say a wizard casts Mind Blank and Improved Invisibility on himself. Does the invisibility now count as part of the caster, or is it considered a separate entity in regards to spells like See Invisibility?

A: Mind blank is not effective against see invisibility (non detection is). Mind blank protects against devices and spells that detect, influence, or read emotions or thoughts. And against scrying, which is magical information gathering conducted remotely. See invisibility is not scrying.

Q: In other words, is the Mind Blanked/Invisible wizard protected from See Invisibility? Detect Magic? True Seeing?

A: No in all three cases.

Which, if you accept these sources as being true renditions of official statements gone past, explicitly denies that Mind Blank would protect you against Detect X spells.

Answer 3

Arguably the biggest defense that an adventurer has against these spells in 3.5 is their relatively short duration (1 min/lvl) - since a caster can't keep them running for long, he or she would have to have some inkling that the adventurer was coming to tell them when to cast those spells. That's one reason why other spells like Alarm exist.

(In Pathfinder, a caster doesn't have a limit on how often they can cast cantrips, so RAW they can just keep casting Detect Magic over and over. If I were GM, I would probably rule that there's some kind of limit on how long you can keep concentrating on spells before you need to take a break, the same way that you can't move double-speed for long distances.)