D&D 5E Alarm Spell vs. Illusionary magic

So, I came across an incident where a forge was mass producing weapons for Tiamat's armies. The PCs went to investigate and found out where the smithing was done only dwarves and wizards were allowed in by the guards. So, they went into an alley cast an illusion spell that allowed each of them to be disguised as dwarves. It was only an illusion though, and so they walked into the factory and I said some audible alarms starting going off as they got halfway into the forge. One of the PCs is a former DM and he looked at me and asked me, really how's that happening?

So, question... Alarm spell vs. a ritual illusory spell? If a magical alarm is coded to go off to non dwarven creatures, should Alarm be triggered by creatures magically looking like dwarven creatures. I made the executive ruling that alarm could detect the make up of the source creature. The wizard in the party uses alarm to guard their camp against anyone that isn't a party member. So, he's telling me that if another wizard use major image or other illusory magic to look like one of the party members, they could come in, cast a Delayed blast fireball while they are all sleeping, then walk out and wait for the fireball to go off?

Should illusions be able to thwart alarm or other detection spells? I think of it this way. Would detect magic detect that the PCs are shrouded in illusionary magic? Would detect good and evil detect the alignment of the PCs or the illusion the PCs are projecting (e.g. if they did a demonic illusion of themselves, would detect good and evil show them as evil-their projection, or good, their innate character development?)

If it's their innate character, I felt alarm would work the same. It would detect the actual creature not the magic the creature is shrouded in. Now, support coming to fight the intruders will need some type of magic or perception check to validate they aren't dwarves that look like other dwarves.... That will be a whole another issue. The PCs at a minimum will get a surprise round to decide what to do as the room fills with enemies looking for the intruders.

Nothing in the alarm spell suggests to me that it relies on visibility, but you could say that it did. If an illusion could fool it, then I'd say invisibility should too. But I'd probably give it the equivalent of truesight as far as its ability to detect creatures, just because the spell description doesn't convey any sense of restriction to me.

To me an illusion effects the senses, namely vision in this case. An alarm spell is magical in nature and my interpretation is that its detection method is beyond the 5 senses. I would have ruled the same as you. The alarm sounds!

However, I could see an argument for taking a perception roll from the Alarm caster versus a deception roll from the caster of the illusion. An alternative might be whichever caster has a higher Spell Attack DC.

I tie alarm magic to the level of the caster.

In this case, ask the question - how could a character of that level determine, 'Is that a dwarf?'

If the alarming magic was cast by a level 3 or 4 caster: A level 4 character doesn't really have any way to answer the question. They can cast detect magic and determine if the dwarf is magic, but that dopesn't tell them if it is a disguise spell or something else. The level 3-4 character will have a lowish perception, so there's a good chance they wouldn't see through a normal disguise. Conclusion, the alarming magic will be fooled by simple disguises and 1st level illusions.

Now, if the caster was 12th level, they have access to much more advanced detection abilities. Higher perception to see through mundane disguises, true seeing-type magic, maybe magic items. Conclusion, the alarming magic will not be fooled by simple disguises and low-level illusion magic.

The level of the caster will, of course, be related to the threat level of the guarded area.

Finally, there is a very important point to be made here: The GM does not follow (a lot of) the rules in the PHB. Just because the PHB doesn't mention an alarm-type spell that detects invisible has no bearing on whether the GM's antagonists can have it. Antagonists don't follow the PHB, so rules on multiclassing and spellcasting and weapons do not apply.

I've had this come up several times in my games. Player says, "He's a fighter, he can't use dodge as a bonus action." I say, "He's not a fighter, he's a monster out of the monster manual, PHB rules do not apply."

The spell they should have used in conjunction with their illusory disguises is Nystul's Magic Aura. Not only could it mask the magic aura given off by their disguise, but it could also make them "read" as dwarves for the purposes of the alarm spell.

Exactly... Greenstone! Not to mention, this is a military outpost that the Rebel Wizards of Thay have taken and have used powerful magics to corrupt the entire town of dwarves. Well, o.k. the red rebel wizards out of desperation made a deal with a demon to amplify some compulsion magics in exchange for some dwarf souls, that allowed them to corrupt the dwarves and basically put them on meth or speed.... The dwarves are all a lot faster at their manufacturing and don't need to sleep or eat anymore... All that was needed was some phylacteries with dwarven souls, some magic from the red wizards, and the demon to amplify the magic.

That said.... The amplification caused the demon to grow more powerful than the wizards and it may have driven some of the red wizards insane... However, their traps and alarms were permanently enchanted into the place, it was meant to be a highly secure location. Now run more by a demon than by a red wizard....

The point being, we're not talking a 4th or 5th level caster.. The group of 5 PCs are 9th level, and the casters that put the alarm spells in place have 7th level spells. Based on the NPC generator I used, it put the Red Wizards at 11th -12 th level casters.

I considered the monster vs wizard dc. But, in the moment, I simply thought - At these levels, and the amplified magic that Demon is providing, the alarm spells would detect their intrusions. It was designed to keep the cult's military personell not under the influence of the speed magic separate from the dwarves so that there were no altercations or issues as the dwarves went insane.

And an executive decision that I still reinforce, if it was a more powerful illusion, then, I'd consider it as an all out winner over the alarm spell. I agree, PHB rules don't apply. Classic examples are monsters with legendary abilities such as the vampire. It can move through peoples squares and does not provoke attacks of opportunity... Not something any PC classes can do on a routine basis. Rogues can disengage as a bonus action, but, that's only away from the monster....

There's a lot I like about 5th edition that favors DMs. That said, I do my best to be fair and/or realistic to the players. I felt guilty for not giving in to them, but in this case, I feel the executive decision I made was a realistic and acceptable reaction to an illusion.

I may have ruled differently since I'm not sure a 1st level spell should be quite that powerful, but you are the DM. You made a ruling on something that doesn't give specific details so your ruling is correct.

Just remember your ruling for the future if it was a simple Alarm spell, it should work the same when and if the PCs use the spell.

I agree with you Oofta.. That's why I was torn.. But other spells cast at higher level spell slots have some interesting effects and become relatively more powerful. So, I was trying to be conscientious of my group as well. They use alarm all the time.. If someone could simply cast an illusion spell to walk into their camp and unless whatever they wanted... They would be extremely pissed at me. So, it's one of those I was ruling based on forethought of what I say stands in the future.....

I see both sides, and I'm not sure there's a clear right answer... I'm leaning towards a home ruled Spell Save DC of the caster of the spell vs. intelligence check. Can the casting of the illusion spell be smart enough to avoid detection by most alarm spells? So the caster of the illusions would roll a D20 and add their intelligence modifier. Something to that extent.

I don't agree a 3rd or 4th level caster casting alarm would be able to detect a 9th level wizard's illusion spell very easily. At the same time, I don't think an illusion spell cast by a 9th level wizard, not focussed in illusion, could completely outwit an alarm spell cast by a 14th level caster... Then, again, there's nothing stating that Alarm spell cast at higher level spell slots above 3rd level gains bonuses to detection... So, I want to capture the intention of the spells. And in this situation it's hard when it's multiple wizards using magic to outwit each other.

MonkeyWrench said:

The spell they should have used in conjunction with their illusory disguises is Nystul's Magic Aura. Not only could it mask the magic aura given off by their disguise, but it could also make them "read" as dwarves for the purposes of the alarm spell.

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This. This is the answer to your query. If the PC's wanted to avoid most "magical" (as opposed to mundane sight based) detection, they should have employed this rarely used spell.

As mentioned, Nystul's Magic Aura is the "disguise from spells" spell.