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My players were going up against a mid-threat mage last session. He cast greater invisibility on himself one round, and the next round, he cast fly. As this silently flying mage was circling the battlefield, one of my players asked a question I couldn't answer rules-wise.

The mage had a Wand of Magic Missiles in his possession, so if Detect Magic was cast, could the outline of the wand be detected through the invisibility spell?

I ruled that it could at the time, because he was a new player and that was pretty good out-of-the-box thinking. But I just wanted to know for future reference.

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    \$\begingroup\$ It should be pointed out that you can't actually use Fly and Greater Invisibility at the same time, since they both require Concentration. \$\endgroup\$ – Miniman Feb 14 '15 at 7:39
  • \$\begingroup\$ @Miniman: Retcon "casting fly" to "drank a potion of flying" and it works. Not like the players could tell the difference, since the mage was invisible at the time. \$\endgroup\$ – ShadowRanger Nov 13 at 16:57
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No, you can't see the creature's aura, but you can detect if there's magic within 30 feet of you, including magic items he might be carrying, though you can't determine the nature of the items. From the text of Detect Magic:

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.

Because the creature is not visible, you cannot concentrate to see their aura nor the auras of any magic items they have in their possession. You cannot pinpoint an invisible creature or object's location with Detect Magic alone.

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    \$\begingroup\$ The presence of the magic item isn't even necessary, as invisibility is magical itself. Detect Magic isn't precise, so you wouldn't effectively "see" the invisible mage with it, but you could tell what "square" he is in, enough to target an AoE spell, or to attack (at disadvantage, but at least in the right location). \$\endgroup\$ – Phil Boncer Mar 27 '17 at 20:58
  • \$\begingroup\$ Detect Magic doesn't even allow you to find the square the target is in. The fact that it isn't hidden does. \$\endgroup\$ – T.J.L. Nov 13 at 13:19
  • \$\begingroup\$ @T.J.L. Unless the creature is hidden, in which case Detect Magic just tells you that there's magic around somewhere. \$\endgroup\$ – Mark Wells Nov 14 at 17:52
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First, as Miniman points out, the evil Mage shouldn't be allowed to maintain concentration on both Fly and Greater Invisibility at the same time.

Second, Jeremy Crawford has ruled (via Twitter) that while you detect the presence of a magically invisible creature within 30' you can't see its aura unless you can see the creature.

Detect magic lets you see an aura only around a visible creature or object, not around an illusion. But you can sense the magic.

So if the Mage had only cast Greater Invisibility, a PC which casts Detect Magic would know that there is some form of magic within 30' but not see an aura for the Mage (or his wand). The sensation of magic could be caused by anything though, including PC magic items or ongoing spells, so this may not be helpful. The PC would not know which square the Mage was in from Detect Magic since he wouldn't see an aura.

Third, the Mage is not guaranteed to be silent via the Fly spell or the Invisibility spell.

However, the Mage should roll a Stealth check (hide as an Action) if he doesn't want the PCs to know where he is. Every time he attacks or makes significant noise (such as spellcasting), he should have to spend a new Action to hide. If a PC's passive perception beats/equals the Mage's Stealth check, then the PC knows what square the Mage is in and can use an attack or spell which does not require sight (many non-AoEs spells with saves require that the target be seen). Attacks (and spells with attacks) will have disadvantage but spells with saving throws will not. A PC can also spend an action to roll an active perception check to try to beat the Mage's Stealth (if their passive wasn't good enough).

Every time the Mage takes any damage they will need to make a Concentration check to remain invisible. The PCs will have to rely on a combination of multiple hits to force minimum Concentration saves and possibly a few big damage (>= 22 damage) hits to force a Concentration save with a higher DC.

A PC which knows which square the Mage is in can:

  1. Attack with a weapon at disadvantage. (Each attack that hits will cause a separate Concentration check.)
  2. Cast an attack spell such as Eldritch Blast or Shocking Grasp or Scorching Ray with Disadvantage. (Each attack that hits will cause a separate Concentration check.)
  3. Attack with Spiritual Weapon at disadvantage (it targets "a creature within 5 feet of the weapon").
  4. Cast Fireball at the Mage's square. The save is rolled normally.
  5. Cast Acid Splash normally (it targets "one creature within range").

A PC cannot:

  1. Cast a spell like Poison Spray or Sacred Flame at the Mage since it targets "a creature you can see."

A PC can probably figure out which square to target by watching where their allies are targeting or being told even if they couldn't figure it out.

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