How do I determine what is "magic" and "bearing magic" for Detect Magic?

Question

I am wondering what "magic" means within the context of 5th edition D&D, particularly as it relates to the detect magic spell. I am familiar with the "The Weave of Magic" and spells slots and such.

The detect magic spell allows you to sense the presence of "magic" and shows a glow around creatures of objects that "bear magic". But what is meant by "magic"? This same question applies to the dispel magic spell and I suspect other spells.

Let's imagine a scenario in which a spellcaster shoots a magical ray of ice at a tree and freezes it. How can I tell what is "magic" here? Is the spellcaster magic while casting it? Is the ray magic (but not a creature or object)? Is the frozen tree now magic, even if encased in mundane ice? Once the ice melts is the water magic? And if the tree has residual damage from the spell, is the tree now magic, or its damaged areas?

I've been playing detect magic pretty loosely and it has worked fine, but sometimes I'm not sure what to highlight in the world. In these examples for example I would not highlight the defrosted damaged tree or water, but maybe I would highlight "residual signatures of magic" or the like. I would likely also highlight a giant cube of ice created through magic, even if it is not strictly described as magical or even described as part of a spell, but merely a consequence of it. Would starting a forest fire with produce flame mean the whole forest fire is magical? I suspect not, but I could be convinced. What about a floating mote of earth being held magically into the air?

I'd like some guidance on what draws the line between magical and non-magical.

Answer 1

In 5e, things that refer to magic only interact with entities that have/are magic as part of their crunch, not fluff. To determine if something is actually magic, rather than described as magic but totally not actually magic, use the rule on page 17 of the 2017 Sage Advice Compendium:

[O]ur game makes a distinction between two types of magic:

• the background magic that is part of the D&D multiverse’s physics and the physiology of many D&D creatures
• the concentrated magical energy that is contained in a magic item or channeled to create a spell or other focused magical effect

In D&D, the first type of magic is [fluff] ... The second type of magic is what the rules are concerned about. When a rule refers to something being magical, it’s referring to that second type. Determining whether a game feature is magical is straightforward. Ask yourself these questions about the feature:

• Is it a magic item?
• Is it a spell? Or does it let you create the effects of a spell that’s mentioned in its description?
• Is it a spell attack?
• Is it fueled by the use of spell slots?
• Does its description say it’s magical?

If your answer to any of those questions is yes, the feature is magical.

Let’s look at a white dragon’s Cold Breath and ask ourselves those questions. First, Cold Breath isn’t a magic item. Second, its description mentions no spell. Third, it’s not a spell attack. Fourth, the word “magical” appears nowhere in its description. Our conclusion: Cold Breath is not considered a magical game effect[.]

(Note that 'described as magic' here means something other than what it normally means, since white dragons, as the entry notes before this passage, are described as magical. It seems like the author means 'in its description' to mean 'in the part of its description where the rules go, you know, its actual description' - but that is contrary to how the authors have claimed you should read spells (etc.) to prevent certain kinds of textual ambiguities and errors from being problems, so make of it what you will.)

This is a helpful starting place, but not sufficient; we still don't know how instantaneous spells with long durations or secondary effects of spells work. For the former, it helps that the rules say about instantaneous spells:

Many spells are instantaneous. The spell harms, heals, creates, or alters a creature or an object in a way that can’t be dispelled, because its magic exists only for an instant.

That means that all the magic after the instant the spell is cast is fluff magic instead of crunch magic. We run this as all the magic stuff happening in a sequence of moments that nonetheless as a whole paradoxically takes up no time. This is important as a naive ruling that such spells do the magic first and then the rest of the seemingly magical effects happen leads to undesirable behavior like being able to fireball people inside an anti-magic field.

For secondary effects of spells, you're just going to have to make a ruling. A summoned creature should probably be magical, grass that caught fire from produce flame and then spread to a house probably should not be. The bullet point that makes them magical is the 'fuelled by spell slots' line, so the more that seems to be true the more it should count, presumably.

Answer 2

Let's tackle your questions separately:

Question: Visual Representation of Magic

The detect magic spell allows you to sense the presence of "magic" and shows a glow around creatures of objects that "bear magic". But what does this mean? This same question applies to the dispel magic spell and I suspect other spells.

Answer: This part is up to your creativity and liking. Neither a more general "glow" or aura around the whole PC nor a specific "magical indication" of a specific part that is magical (e.G. one weapon is glowing) will make or break your game. I think depending on the kind of magic that is being used you could describe the magic as a light aura (cleric), dark aura (warlocks, evil guys etc), nature themes (green/brown/blue) aura (druids, rangers) and so on, if you want to give your PCs a bit more input.

Question: Spellcasting and Spellcasting Duration

Lets imagine a scenario in which a spellcaster shoots a magical ray of ice at a tree and freezes it. What is "magic" here? Is the spellcaster magic while casting it? Is the ray magic (but not a creature or object)? Is the frozen tree now magic, even if encased in mundane ice? Once the ice melts is the water magic? And if the tree has residual damage from the spell, is the tree now magic, or its damaged areas?

Answer: As a general rule of thumb I'd look at the spell's duration

• If the spell's duration is instantaneous the magic is recognizable in the moment of casting. Taking the Fireball spell as an example: During the moment of casting the wizard can be identified as using magic and maybe already the air between his hands starts to indicate a magical evocation. When the fireball is released the fire is magic fire and not normal fire, thus it is recognisable as magical. This has been confirmed indirectly by how fireball works underwater, see this sage advice podcast on underwater fighting. In short: A magical fireball does not follow the laws of physics and is distinctly different from non-magical fire. After the fireball is gone it might have caused non-magical fire that follows all laws of physics.
• If the spell has a duration and thus the effect lasts longer (as in your "ray of ice" example) the magic is recognizable for the whole duration of the spell.

Note: Regarding your example of freezing something: Which spell is that? In case you are referring to Ray of Frost following RAW you cannot freeze anything with it. I would handle Ray of Frost the same as Fireball. Regarding the spell that you described I would judge that, if it is instantaneous the ice is nonmagical or, if it has a duration, the ice is magical.

Let's dissect the last part of your question further:

I've been playing detect magic pretty loosely and it has worked fine, but sometimes I'm not sure what to highlight in the world. In these examples for example I would not highlight the defrosted damaged tree or water, but maybe I would highlight "residual signatures of magic" or the like.

"Residual signatures of magic" is a wonderful way of describing gray areas. If it is working at your table, great!

I would likely also highlight a giant cube of ice created through magic, even if it is not strictly described as magical or even described as part of a spell, but merely a consequence of it.

Following my above explanations I, personally, would not describe those as magical. If you want to highlight the "weirdness" of ice damage or something alike you could simply state that "the damage type does not match any mundane effect that the PCs would attribute to the current environment or its inhabitants". Otherwise your PCs might feel inclined to use magic to repair or dispel the lingering magical effect.

Would starting a forest fire with produce flame mean the whole forest fire is magical? I suspect not, but I could be convinced.

You are right, it is non magical. Following RAW the spell does not allow causing damage unless used as a ranged attack which ends the spell, see Produce Flame. I'd interpret that the flame only hurts and ignites when used as a ranged attack and that effect is instantaneous (because the spell automatically ends afterwards). Following my above explanation instantaneous only triggers natural fire, not magical. If you houserule that the spell can be used to ignite materials for the whole duration I'd still rule that the flame itself is magical and causes the ignition of other materials that then burn non magically. Thus, the source of the fire was magic, but following RAW as soon as the forest caught fire the fire is nonmagical.