What are the results of a detect magic spell in regards to some of Barovia's magic features in Curse of Strahd?

Question

It was our first session of Curse of Strahd, and I used the Creeping Fog adventure hook to get the party to Barovia. Immediately, they grew suspicious of the mist and cast detect magic.

I was able to reveal that the mist was of a magical nature, but when they pressed me to reveal the school of magic, I had to simply say that it was unclear.

Does anyone know what detect magic would actually reveal?

When they finally reached the Village of Barovia, they came upon Death House and grew suspicious.

One party member was concerned that the children outside, Rose and Thorn, were illusions. They are. She cast detect magic. I had no idea how to deal with this, so I set the DC at 20, figuring Death House cast a pretty good illusion, and in a meta-game sense, I was worried if they discovered the illusion, they wouldn't go through the house, and I'd be stuck. She rolled very low, so I was comfortable and I think they all were, too. But what if she had rolled a 20? At what DC would you set Death House's illusion?

Are there other instances in CoS where I will have similar difficulties, and if so, what should I do?

Answer 1

Detect Magic does what it says it does. I don't see anything in Curse of Strahd that affects this particular spell explicitly to change that effect.

For the duration, you sense the presence of magic within 30 feet of you. If you sense magic in this way, you can use your action to see a faint aura around any visible creature or object in the area that bears magic, and you learn its school of magic, if any.

In the case of the children there would have been no roll required on your player's part. She would cast the spell, realize there was magic present and then used her action to know that the children had magic on them, in this case of the school of Illusion. When facing a Wall of Ice (cast by the spell) it would reveal Evocation. Instantaneous effects may or may not have a lingering effect, DM's discretion.

The system dances around the issue that an illusion is not really an "object" or a "creature" so the DM is free to make that ruling when resolving targeting of spells. Keep in mind by doing so you invite meta-game thinking and when they can't target the creature or object they might draw conclusions based on that.

As a DM when you have a published module and they do not indicate explicitly what spawned an effect you will have to find the closest effect in the system and compare that to the most likely source in the module, or create one to determine if and when to assign a school when resolving Detect Magic.

Keep in mind that, unless she is a Warlock, she has to burn a slot each time she casts it or spend 10 minutes doing so as a ritual. This is an opportunity cost on time and prepared spells. Keep in mind also that the spell can be blocked by a number of things.

Nondection, Nystal's Magic Aura, 1 foot of stone, 1 inch of common metal, a thin sheet of lead, or 3 feet of wood or dirt.