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Consider the Night Hag. The Night Hag has "constant detect chaos, detect evil, detect good, detect law, and detect magic."

However, each spell reveals additional information for each round "you study a particular area or subject." Detect Magic, for example:

1st Round: Presence or absence of magical auras.

2nd Round: Number of different magical auras and the power of the most potent aura.

3rd Round: The strength and location of each aura. If the items or creatures bearing the auras are in line of sight, you can make Knowledge (arcana) skill checks to determine the school of magic involved in each. (Make one check per aura: DC 15 + spell level, or 15 + 1/2 caster level for a nonspell effect.) If the aura emanates from a magic item, you can attempt to identify its properties (see Spellcraft).

Would a rogue wearing magic items be able to sneak up on an inattentive Night Hag? (Assuming his Stealth is sufficient.)

Unlike continuous True Seeing, which is entirely passive, Detect spells seem to need to be activated. In other words, does a creature with continuous Detect Magic need to consciously choose to begin using the ability before it has any effect? If not, how do you handle the fact that the area of the spell is a cone, and not a radius? And if so, what is the difference between "continuous" Detect Magic and using it "at will"?

(Unless something in Pathfinder overrules it, D&D 3.5e answers are good too.)

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up vote 15 down vote accepted

Since all of the Detect spells have a duration of Concentration, it seems to me like you have to concentrate to gain a benefit from them. If the Night Hag concentrates for 1 round, she can use any of her detect spells without having to cast it, but she doesn't get any information without concentrating. The difference here between continuous and at will is that the continuous version doesn't provoke an attack of opportunity. I don't think there's a solid rule on this, but I feel like this solution fits the normal behavior of the spells best.

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+1 For the difference between "at will" and "continuous". – dlras2 Mar 10 '12 at 6:35
@Yandros: That's actually not true. Making a reactive Concentration check is a free action, but concentrating on an active spell with a duration of Concentration is a standard action. See the "Concentration" section here: – DuckTapeAl Mar 10 '12 at 20:39
@DuckTapeal Hmm, I don't think the part of your answer about "at-will" and "continuous" works. The SRD says "A spell-like ability has no verbal, somatic, or material component [...] The user activates it mentally." and "Spell-like abilities cannot be used to counterspell, nor can they be counterspelled.", so "can't be countered" and "no one knows that she's using it" counts for both. The only difference I can see is "Using a spell-like ability while threatened provokes attacks of opportunity." – Yandros Mar 10 '12 at 21:28
@Yandros Aha. Well spotted, yourself. :) – DuckTapeAl Mar 10 '12 at 21:42
@Yandros—Kind of a separate question, but does it give any reason why does a spell-like ability with no no verbal, somatic, or material component provoke an attack of opportunity? – dlras2 Mar 11 '12 at 18:32

We have always played it that you always get the first round benefits for "always on" and can choose to concentrate, getting the 2 and 3 round benefits for 1 and 2 rounds of concentration, respectively. However, "at will" is different, you get the same benefit that anyone who casts the spell would, but you can start your concentration at any time.

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Yes she would have to concentrate on it for it work. That's why Arcane sight is SOOO much better then detect magic, because it works passively within sight range.

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