D&D 5E Alarm Spell vs. Illusionary magic

[Rocksome]

EDIT: As a side note, this where as a DM I would have suggested that those players with the Arcana skill, make a roll and given them the opportunity for them to "know" the magical theory that I was operating under, with a successful roll I would say "Remembering back to some of your earlier lessons, you remember that magical alarms do not work on visual cues, but on magical divinations and a simple illusion is unlikely to bypass it".

I believe that as a low level spell, Alarm doesn't have the power to determine anything for itself. It can't magically sense Dwarves and Wizards. Otherwise you could use it as a detector for anything. The conditions "people who didn't try and kill me last night", or the condition "People who aren't plotting to overthrow the king" would allow the PCs to work out any secret with a single low level spell.

What defines a Wizard? Belonging to the class? What about multi-class characters? What about someone with the Magic Initiate feat?

I don't think the spell should have any extra-ordinarily powerful divination magics to determine if it goes off. Straight up visual sensing seems far more appropriate. I've interpreted the clause "designate targets who don't set it off" as people you know and can name (not theoretical groups of people: vegans, pessimists, politicians) . And I'd say that without any magical senses a disguise of any sort would fool it.

Alternately, If you prefer a magical sense I would still explicitly require them to state the individuals who don't set off the alarm, and their magical signature gets read and ignored. In this case, I'd allow Nystul's Magic Aura to bypass it.

At the end of the day it is open to interpretation. I just think by allowing phrases like "wizard" to bypass it you're asking for a headache.

 

[jaelis]

Did the players know there was a magical alarm set up?

 

[MarkB]

Just to point out, the Alarm spell isn't able to detect creature types, and isn't fooled by illusory appearance because it doesn't rely on appearance. It triggers if any Tiny or larger creature enters the area other than those specifically designated at the time of casting.

It's not spelled out, but I'm pretty sure it's more a matter of the designated creatures being 'tagged' when the spell is cast rather than the spell actually looking at a creature and deciding whether it matches the description of a designated 'safe' creature. Any creature not bearing the 'tag' will trigger the alarm.

What would work in these circumstances is Magic Mouth. That spell lasts indefinitely, and can be programmed to react to a fairly specific circumstance - "if anyone other than a dwarf steps within 30 feet" is well within its remit. However, you really are back to square one at that point, as the spell description says nothing of how it would perceive an illusory disguise.

 

[Saeviomagy]

My first port of a call with a lot of spells like this is to look back at the 3.5e version of the spell.

In 3.5e, alarm couldn't discriminate between creatures, it only had the option to not trigger if a creature gave a password before entering the area. 4e just kind of took an editorial wrecking ball to that without really considering the consequences. You might consider reverting to that version of the spell, in which case you've added an extra step to your infiltration of "find out the password", which to me is much more satisfying than blurry rules about whether it can determine your dwarfishness through unspecified sensory capabilities.

Also looking at magic mouth, which is a similar sort of spell: it requires 'visible or audible' cues within 30ft. To me, that means it would fall for illusions and disguises, and since it lacks any capability to make an investigate check, it would automatically be fooled. If I was going to say that alarm is able to be more subtly directed, magic mouth would be the model I would use.

 

[Gadget]

I believe that as a low level spell, Alarm doesn't have the power to determine anything for itself. It can't magically sense Dwarves and Wizards. Otherwise you could use it as a detector for anything. The conditions "people who didn't try and kill me last night", or the condition "People who aren't plotting to overthrow the king" would allow the PCs to work out any secret with a single low level spell.

You raise some good points, but this where DM adjudication needs to play a crucial part. IMO, Any DM worth his/her salt would not allow the Alarm spell to be turned into a free find-out-stuff-about people spell. I would put this in the old bag o'rats category ("the ability says I get to do this when adjacent to or I damage a 'hostile' creature; I keep this bag of rats on hand that I treat poorly to fulfill the requirement").

Looking at the spell text specifically it says: "When you cast the spell, you can designate creatures that won’t set off the alarm". Ipersonally would interpret this as naming specific creature(s) (Fred, Jane, and my cat Mr. Wiskers), or a specific category of creature (goblins, fiends, cats, and yes: dwarves). Maybe I would further allow sex or adult/child designations. Something along the lines of: "those of us present here at the casting" would be acceptable as well (leaving open the possibility of an invisible/disguised adversary accidentally being included). As such, something like 'wizard' would be pushing my interpretation of the spell. Though I am amendable to the 'Rule of Cool' and could possibly be convinced otherwise.

Here is a question: If the party had instead Polymorphed into dwarves, would the disguise have worked? I'm inclined to say yes, based on the spell description of creatures, assuming whoever cast the spell did it that way instead of specifically designating each enslaved dwarf individually.

What if the spell was cast to allow in only the King's Vizier, could one Alter Self/Polymorph oneself to that specific form and fool the spell? What say you?

 

[Shiroiken]

Did the players know there was a magical alarm set up?

Also, did the Alarm spell exist before the illusion spell? I've seen this too often when the players find a great way around a challenge, the DM invents a sudden "fix" to the encounter (effectively making player inventiveness worthless).

 

[tk32]

I had not seen that spell, yes, only 2nd level spell too.. This could have been used to make all the alarms appear as non magical also on the enemy's side.