If a person is made invisible through the use of an invisibility spell, would the detect magic spell show where they are? My first thought is that it would, because it's an ongoing magic spell. I feel like the person casting detect magic would see some kind of "glow" where the invisible person is located.
The complete rules for the effects of Detect Magic are: "One of your senses is briefly attuned to magic. The GM will tell you what here is magical." So, up to your GM, or more broadly, to your play group, to decide how Detect Magic works in your game. It doesn't even have to be sight-based; maybe you smell magic.
For what it's worth (very little), as a GM I would tell the player that there's something else magical around here, maybe it was over there in that corner? Hard to be sure.
One of the GM principles in DW is Ask questions and use the answers. A good GM would probably ask the wizard how their detect magic works, and make a note to use that detail to fill the adventurers' lives with more adventure.
"The Zaphod Beeblebrox?" "No, I come in six-packs."
There isn't such a thing as an invisibility spell, or a detect magic spell. There is Wizzrobe's detect magic spell and Clericsdottir's invisibility spell (which she poached with her human racial). No one else in the world gets the playbook text unless they've got one of the playbooks.
Now, it's possible you may be legit be looking at a player v. player situation here, but those are kind of all jacked up to begin with, so let's start out by looking at the player v. nonplayer stuff.
And, I mean, that kind of thing is going to come up eventually. Have things got magical senses? Sure. Is there stuff that turns invisible? Almost certainly. How do they interact?
How do you want them to interact?
Playbook Detect Magic vs. A Thing What Goes Invisible
Detect Magic doesn't have to be a sovereign counter to every sneaky move in your sneaky book. Think about the situation, about the question "how much like the magic you're used to detecting is the effect producing the invisibility?" Decide whether Detect Magic is going to provide:
- an advantageous progression of events, shifting from a risky place to a point of knowledge and control, or:
- an ordinary progression, keeping momentum going over a risky situation, or:
- a disadvantageous progression, requiring additional risks in a desperate attempt to be useful
We'll assume for purposes of the examples here that Wizzrobe is a proud* graduate** of the Imperial Wizard Academy.
*oh lord yes **oh lord no
An Advantageous Progression - Magic You Know:
"Yeah, there's a small horde of freshly washed zombies swathed in a textbook example of the Finis Diffractor. Like, too textbook. What kind of imperial wizard wants to sneak zombies into the county seat?"
(It's actually the lich who founded the Imperial Wizard Academy in the first place and then left to pursue other interests.)
This both reveals the invisible thing and provides some additional information on top, but there are other sorts of advantages, like seeing how you might be able to dispel the invisibility.
An Ordinary Progression - Magic You Know About:
"Yeah, you could probably lay eyes on these nettlesome pixies if you cast it. But given how you've interacted with faerie magic in the past it's probably going to be like holding a handful of sparklers right in front of your eyeball. At the very least you can give Fightgar somewhere to swing. Still doing it?"
So, yes, casting Detect Magic reveals the invisible thing and lets it be acted upon, not necessarily at a significant risk to yourself but in such a way that the spotlight moves off you for the moment and over to somebody else.
A Disadvantageous Progression - Magic Undreamt Of In Your Philosophy:
(At some point a wolf took a shadow-lover. Now their spawn hunts the party through a dark forest at midnight.)
"Some of these shadows are alive. You're very certain of that. But they're all tangled up with the shadows that aren't and it's hard to say where everything begins and ends. If you wanted to hold your ground and Discern Realities, well, you'd need a willing guard, but you could certainly use your enhanced senses to find out."
Actually being able to successfully use Detect Magic to reveal the invisible thing is going to mean putting yourself at risk and/or making another move.
Playbook Invisibility vs. Arcane Senses And/Or Sensibilities
In a similar fashion, it's probably worth thinking about what kind of weird detectorage abilities are out there when someone's moving around under cover of invisibility. The operant question is "how do they know you're there if they can't see you?", and invisibility is supposed to provide you with an advantage by default, but according to circumstances you may want it to provide:
- a sovereign advantage: yeah, there are patrols, but only a couple of them have hounds which might sniff you out, and as long as you don't get cornered by those you can move freely.
- a situational advantage: oof, the corridors are studded with runeplates. You'll still need to watch your step in here, this isn't called ingravibility.
- a questionable advantage: great, this is where the duke's parked the elven nobles and their delegation. Didn't Wizzrobe say all elves could kind of sense magic? You're going to be more "lightly fogged" than "hidden".
Playbook Detect Magic vs. Playbook Invisibility
So. It has come to this.
Well, I hope it hasn't come to exactly this, since those are both Wizard spells and while you're concentrating on invisibility you can't cast other spells.
But if they're spells for two different people, you can still start out by asking, exactly how related are these magics? Does one side have a clear advantage?
"Wizzrobe, you've been letting Shanksworth crib off your lecture notes to learn magic. Would you really let him learn a Finis Diffractor you couldn't just spot without trying?"
"Clericsdottir, we've played your Invisibility spell as granted by Gerak the Scarred, patron of the downtrodden, and said what it really does is make people think you're beneath their notice. Would magical senses actually help Wizzrobe notice you?"
If there is no clear advantage, one fairly common way to deal with a PC-on-PC conflict is for one player to act and the other to roll +bond to interfere.
So in this case, probably the best way to play it is that the Invisibility side is going to Defy Danger to either evade detection and get away or act decisively before their advantage is gone, and the Detect Magic side, assuming they can get Detect Magic off, will roll to interfere.