Detect Magic shenanigans

[Crinias]

Divinations are an important part of being careful and prepared for danger (essays could be written on the subject), and the simplest, most humble spell of this powerful school of magic is Detect Magic.

Despite some limitations (particularly having to concentrate on it), this spell can be a lifesaver, used to learn if there's a magical trap somewhere (or abjurations in general), knowing which spells your opponent has cast on itself and other such things.

However, I recently came upon a conundrum. Me and my group regularly use Detect Magic and we've run into a few situations where Supernatural effects were important, either mechanically or plot-relevant.

I realized only later that Detect Magic and for that matter Spellcraft only specify spells and magic items. So, even though Supernatural abilities clearly are 'magical', such as a dragon's breath or a medusa's petrifying gaze, and they go away in antimagic fields, they can't actually be detected by Detect Magic.

Knowing this earlier could have spared me as DM a lot of grief.

What is the consensus on this? Has this ever come up before on one of your games or is there another way to detect supernatural abilities?

[Psyren]

Sure they can. Detect Magic actually detects "auras," not just spells or items. This aura is dependent on caster level, and all supernatural abilities have one - it's equal to the owner's HD unless otherwise specified. Therefore you would use the "nonspell effect" formula (15 + half caster level.)

If a supernatural ability is instant (e.g. a dragon's breath) and you came along after the fact, you would use the "lingering aura" rules to determine what it was and how long ago it was used. So a great wyrm's breath would be detected longer than a young adult's, following the rules above.

[Duke of Urrel]

I think I have something to add to what Psyren said.

When a magical creature activates a supernatural ability to bestow magic upon itself, this magic manifests itself as a magic aura that is detectable by means of the Detect Magic spell, just like any other active magic. For example, when a pixie turns invisible, it carries a detectable magic aura, just as if it had bestowed this effect upon itself by casting a spell. However, a pixie can suppress its invisibility as a free action, and soon afterward, its residual magic aura dissipates. When even the residual aura is gone, the Detect Magic spell can no longer reveal anything magical about the pixie.

When a medusa has "turned on" its petrifying gaze, I believe the Detect Magic spell eventually reveals a magic aura around the medusa's eyes ... in theory. But as a practical matter, focusing on a medusa's eyes long enough to see this magic aura is very likely to get you turned to stone. A medusa can "turn off" its gaze, and a few minutes after it does so, its eyes have no more residual magic auras around them, and the Detect Magic spell can no longer reveal that they are magical. But for a short time after a medusa has recently turned off its petrifying gaze, you might be able safely to detect residual magic auras around its eyes – maybe in time to take evasive action, if you prefer.

At least, this is how I understand the rules. I am interested to know whether others interpret them differently.

A similar rule applies to spellcasters. You can't use the Detect Magic spell to find out in advance whether somebody can cast spells or not. But if somebody has cast the Undetectable Alignment spell on herself, you become able to see a magic aura around her. Even after this spell expires or is dismissed, it remains detectable for a few rounds afterward as a residual aura.

In my own house jargon, I say that the Detect Magic spell can detect only 1. actual or 2. residual magic, but not 3. potential magic.

1. Actual magic is detectable magic that can be in either one of two states: captive magic (that is, the magic contained in a magic item or a magical trap that awaits being triggered) or active magic (that is, an ongoing effect, like Invisibility or Bull's Strength).

2. After actual magic expires, it becomes residual magic for a limited time before it dissipates and becomes completely undetectable again.

3. Every magical creature (that is, every creature that has a supernatural ability, a spell-like ability, or the ability to cast a spell) possesses potential magic at all times, but the Detect Magic spell cannot detect this magic until the creature "actualizes" it in some way.

The Arcane Sight spell and the Greater Arcane Sight spell enable you to detect spellcasting ability and spell-like abilities in a creature, which means that these spells enable you to detect two types of what I call potential magic. I don't know of any spell that reveals whether a creature has supernatural abilities before it activates (or has recently activated) one of them.