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The Alarm spell in DnD 5e specifies:

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.

It does not specify that the caster should be able to see the target area (unlike many AOE spells).

Does this mean that it works through walls and doors to cover any 20'x20'x20' area I choose in range, irrespective of the structures within that area?

RabidMutant
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  • Could you please edit the title or the body to make them ask the same question? For now we can't answer "yes" or "no" to both "the spell have limits" and "the spell works through walls", which can be a source of confusion. – enkryptor Feb 15 '22 at 06:52
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    Updated question to the best I can manage; feel free to suggest an alternate if there's a better one. – RabidMutant Feb 16 '22 at 10:01

1 Answers1

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A Clear Path to the target

To target something, you must have a clear path to it, so it can't be behind total cover. If you place an area of effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.

Areas of Effect

Every area of effect has a point of origin, a location from which the spell's energy erupts.

...

A spell's effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn't included in the spell's area.

...

You select a cube's point of origin, which lies anywhere on a face of the cubic effect. The cube's size is expressed as the length of each side."

A diagram to illustrate enter image description here

Eddymage
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  • Hmmm. That's about targeting 'something'. Is an area a 'something', and if so, is it sufficient to see the edge of the area (in this case a wall or a door)? Specifically it says 'If you place an area of effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.' and a cube can have a point of origin anywhere on any face. – RabidMutant Feb 15 '22 at 04:01
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    @RabidMutant: https://www.dndbeyond.com/sources/basic-rules/spellcasting#AreasofEffect "Every area of effect has a point of origin, a location from which the spell's energy erupts." ... "A spell's effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn't included in the spell's area." ... "You select a cube's point of origin, which lies anywhere on a face of the cubic effect. The cube's size is expressed as the length of each side." –  Feb 15 '22 at 04:03
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    Thanks! That does seem to cover it. – RabidMutant Feb 15 '22 at 04:08
  • So...all of those spells that say "that you can see" are just wasting the ink? – RabidMutant Feb 15 '22 at 04:10
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    @RabidMutant The intent is to remind people of the rules, this has caused some problems in the past with the helper text being read as additional rules, or people trying to use the existence of helper text on spell A to prove something about spell B. ‍♀️ –  Feb 15 '22 at 04:20
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    A clear counterexample of targeted locations always needing a clear line of effect is the Clairvoyance spell, which can explicitly target locations up to a mile away (as long as they're familiar to you), or blocked from your view (such as the far side of a door). – Blckknght Feb 15 '22 at 05:59
  • So you must have a clear path to the point of origin, not the whole area. Therefore, some parts of the area can be behind a wall, etc. Is it correct? – enkryptor Feb 15 '22 at 06:27
  • @enkryptor Correct. Imagine you are shooting a fireball through the window of a house and it explodes inside hitting everything in the room - even though you can't see every part of the room's interior from where you are standing outside. –  Feb 15 '22 at 06:31
  • @Blckknght I'm not sure how to interpret Clairvoyance - maybe you really do need a clear line of sight. However the rules are crystal clear and explicit; "To target something, you must have a clear path to it". –  Feb 15 '22 at 06:34
  • @Non-humanPerson so the answer is actually "Yes" then? The spell does work through walls and doors. – enkryptor Feb 15 '22 at 06:50
  • By saying "No", did you mean an answer to "Does the Alarm spell have limits on the area affected" or the question body "Does this mean that it works through walls and doors"? – enkryptor Feb 15 '22 at 06:53
  • @enkryptor lol good point, I removed the "no" to let the text speak for itself and avoid confusion. –  Feb 15 '22 at 06:54
  • I think that this answer could be improve by adding some sentences explaining the rules and not just citing them, even if their application seems quite straightforward. – Eddymage Feb 15 '22 at 07:50
  • @Eddymage I'm not sure what more I could add. If you've got some ideas go ahead, I feel like I'd just be repeating the same thing again in different words. –  Feb 15 '22 at 07:50
  • "repeating the same thing again in different words": yep, that's an explanation! :-) – Eddymage Feb 15 '22 at 07:53
  • @Eddymage I'd rather not then, it seems clear to me and looks like OP is understands it! Best to let the rules speak for themselves whenever possible. –  Feb 15 '22 at 07:55
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    @Non-humanPerson I found a discussion that covers "that you can see" and why it is important, here https://rpg.stackexchange.com/questions/62840/do-you-need-line-of-sight-to-cast-spells-on-someone ; the critical difference is that line-of-sight might exist but the target is not visible. So if it does not say "that you can see", then only an unobstructed path is needed (mist or magical darkness can be ignored).

    Basic rule seems to be: unobstructed path ALWAYS required (unless explicitly excluded); visibility requirements depend on spell description.

     – RabidMutant Feb 16 '22 at 10:05