Can Alarm be used to detect hidden creatures?

Question

Alarm can be cast over a 20×20×20 ft (or smaller) area within a range of 30ft, and has a casting time of 1 minute.

Until the spell ends, an alarm alerts you whether a Tiny or larger creature touches or enters the warded area.

I recently used this spell by casting and ritual casting it on chests, to see if they were mimics. It ended up backfiring, technically there was a mimic, but it was a tame one that was a part of a magic item.

Anyway, from my understanding of the spell, this should work, and I thought it was a pretty neat trick to test chests. However, it seemed to be frowned upon, and I was told I could only use it in such a way this one time.

I just want to know whether the use of Alarm to detect whether a creature is in an area, is a valid use of the spell according to RAW. Of course, exempting Tiny Beasts when you cast the spell, so that you don't get screwed over.

Answer 1

No. It will work to detect any creatures that intrude the warded area by entering or touching it; this is not such a case though.

TL;DR - The spell is Alarm, not detect creature, nor detect life. However! Magic Mouth is the (2nd level) spell you're looking for to set up this sort of "scanner" in game as you can specify such a trigger "when within 30 feet of a mimic" etc.

As described, Alarm is a 1st Level spell with the following effects (emphasis mine):

You set an alarm against unwanted intrusion. Choose a door, a window, or an area within range that is no larger than a 20-foot cube. Until the spell ends, an alarm alerts you whenever a Tiny or larger creature touches or enters the warded area. When you cast the spell, you can designate creatures that won't set off the alarm. (Details on the type of alarm and triggered affects follow)

This includes invisible creatures, mimics, even creatures pretending to be objects (so long as they are still considered creatures) BUT only if they intrude via entering or touching the warded area.

• If you ward a window or door, and someone touches the door after the alarm is set. Your alarm triggers as soon as they touch (or enter it if left open for some reason).

• If you ward an area (20ft Cube) the alarm is triggered if a creature enters or touches the area (same as above).

So if I ward an area with statues already in it trying to see if one of them is a creature hiding in plain sight, will my alarm trigger from the creature touching the area?

This is where the debate is.

1. Do native occupiers of the area considered, "touch" the area?
2. Are they considered to be intruding?

Answer to 1. Technically they already were touching the area prior to the alarm being set, so "touching" the area they still are.

Is there a difference though between "first touching" and "already touching?" I believe so, example:

If you were to be advertised, "If you sign up now, we have a special offer!" and you explain, "I've been signed up this whole time, where's my special offer?" It isn't a surprise to find that there is a difference between establishing a new status, and continuing an already established status when they say no special for you. It would stand to clarify "is touching" or "first touches" but simply "touches" is ambiguous enough to look to context for clarification.

(Others too have mentioned, there is another unspecified difference between "the status of two things touching", and "the action of reaching out and touching something.")

Since this isn't clarified directly, we can look to the description for context to read the intention of the effect.

You set an alarm against unwanted intrusion.

So the alarm is specifically against "intrusion", defined as:

putting oneself deliberately into a place or situation where one is unwelcome or uninvited.

Which a non-moving, already in place creature that you approach is not doing. To intrude requires a state change of "out of area" to "in area" which these mimics would not be doing.

Further reasoning for the interpretation of intrusion:

Are my bones intruding my body? My bones are currently touching my muscles, my skin, and is indeed inside my body etc... but they would not be considered to be intruding."

So the Answer to Number 2 in the case of scanning for non-moving, hidden mimics; unless they move into (or are moved into) an alarmed area, intruding they are not.

Answer 2

Yes, this will work.

Alarm states:

Until the spell ends, an alarm alerts you whenever a Tiny or larger creature touches or enters the warded area. When you cast the spell, you can designate creatures that won't set off the alarm. You also choose whether the alarm is mental or audible.

Because the alarm is set off the moment a creature touches the warded area, if a creature you have not designated is touching the area when you cast it, the alarm will be set off immediately upon the spell being cast.

Personally, I see no reason to disallow this use of the spell, it is definitely within the scope of rules as written, and requires either a spell slot or time investment, both of which are going to be valuable resources when exploring a dungeon. Waiting around for ten minutes to check for hidden creatures is a great way to have those hidden creatures decide to get the jump on you. At the very least, this method seems far less cheesy than this one: Can you prevent players using cantrips to tell whether a creature is disguised as an object?

Answer 3

Yes but it is always a DMs call to make.

As you quoted the spell mentions:

a Tiny or larger creature touches or enters the warded area.

Hidden creatures are still creatures and are thus detected by the spell because they will be touching the area as soon as the spell takes effect. To me this is a pretty straight forward reading of the spell with no mental gymnastics required.

From the perspective of the DM I can see why this might be frustrating and why they resorted to mentioning that it might not work going forward. They might be afraid this one ritual spell will 'ruin' every encounter that relies on a hidden creature. It is their call to make but if you want to change their mind I would focus on the spell's casting time of 1 minute or 11 minutes as a ritual. A lot can happen when you are busy casting the spell, other creatures can appear or the hidden creature notices what is going on and decides to attack before you can finish, perhaps causing you to be surprised for a round because you were busy casting. Approaching the situation this way can remind the DM this one spell is not negating fun encounters but simply changing them to different probably equally fun ones.

Answer 4

No

The spell detects a creature when it “touches or enters the area”. A creature already in the area does neither of those things.

Answer 5

RAW, the ground and air are in the spell's warded area. It is touching the ground, and the air around it. So it is touching the warded area. So it should set off the alarm spell as soon as you cast it.

The spell would go off with tiny beasts though, since they are, by definition, tiny or larger. You could cast the spell so that the bottom of the spell's area is off the ground so that tiny creatures don't get detected, but then something in the know could just crawl in under the radar.

Honestly, if you want to know if a chest is a mimic or not, you don't need magic: just attack it with an axe to see if it fights back. This is assuming your character has a reason to be paranoid about mimics.