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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?

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  • \$\begingroup\$ 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. \$\endgroup\$
    – enkryptor
    Commented Feb 15, 2022 at 6:52
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    \$\begingroup\$ Updated question to the best I can manage; feel free to suggest an alternate if there's a better one. \$\endgroup\$
    – RabidMutant
    Commented Feb 16, 2022 at 10:01

1 Answer 1

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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.

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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

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    \$\begingroup\$ @RabidMutant: 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." \$\endgroup\$
    – user73918
    Commented Feb 15, 2022 at 4:03
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    \$\begingroup\$ Thanks! That does seem to cover it. \$\endgroup\$
    – RabidMutant
    Commented Feb 15, 2022 at 4:08
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    \$\begingroup\$ @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. 🤷‍♀️ \$\endgroup\$
    – user73918
    Commented Feb 15, 2022 at 4:20
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    \$\begingroup\$ 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). \$\endgroup\$
    – Blckknght
    Commented Feb 15, 2022 at 5:59
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    \$\begingroup\$ @Non-humanPerson I found a discussion that covers "that you can see" and why it is important, here rpg.stackexchange.com/questions/62840/… ; 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. \$\endgroup\$
    – RabidMutant
    Commented Feb 16, 2022 at 10:05

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