How to handle a warlock with the detect magic invocation?

Question

My group has a warlock that took the eldritch invocation eldritch sight which allows them to cast it without consuming a spell slot.

This is causing some concern for me as the DM as I currently have the party in a haunted mansion that deploys illusions as a way to mask traps, or hidden treasures, etc.

The spell says:

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.

Do I have to tell them it's illusion magic? How does this work? I find this breaks my storytelling and I would love some advice on how to handle it.

Edit: I don't want to screw over the warlock, I just want tips on how to handle it so that every time I try to use an illusion they don't automatically detect it for free. Thanks everyone.

Answer 1

Eldritch Sight (PHB 110) allows you

cast detect magic without expending a spell slot

An illusion is magic, so they will detect its presence automatically within 30', and will also be able to use an action to see the aura around the specific source and determine the school of magic, per the Detect Magic spell. Note, the illusion itself won't show an aura, but the trap trigger should, if the warlock is within range of it when it is triggered.

According to Jeremy Crawford (lead rules developer on 5e), when he was asked on Twitter

Would Detect Magic show an aura around an illusion?

His response was

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 the warlock would be able to sense the presence of the magical trigger or illusion source and could detect that it has an illusion school aura if he can spend an action. If the trap is already triggered (assuming the illusions are cast by a trap) then the illusion itself will set off the magic "spidey sense" but wouldn't have an aura. The warlock would have to be within 30' of the trigger/source.

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There are a few ways to confound a player using Detect Magic, though you really ought to accept this PCs capability and use it for good story moments, rather than trying to shut it down, which will only frustrate the player.

1. There are so many things radiating with magic that the PC can't distinguish between illusions and just ordinary things that have some residual magic aura without spending a lot of time (actions). Presumably whoever created this haunted house knew of Warlocks and this ability, so they planned accordingly. So give the PC lots of things to investigate, eventually they will get bored and walk into a trap. It is also possible that the use of Nystul's Magic Aura could mask the magical aura of the trigger/source as another school of magic. I would postulate that a warlock who uses Detect Magic frequently would habituate to the magic used by the party (the fighter's magic sword, the thief's magic ring, etc) and could detect a new magic source, lest Detect Magic become fairly useless unless the warlock is alone and divests himself of all his other magic items.

2. Using this magical ability attracts negative attention. Maybe the house responds aggressively to use of magic, the walls close in, things fly at the magic user, etc. This will force the PC to limit use of the ability.

3. Accept it and let the PC detect all the illusions and their source if he is close enough. But remember that things can block Detect Magic, like 1 foot of stone, 1 inch of metal, etc. So chests and such could be behind secret, non-magical doors.

4. The range is also only 30', so things farther than that won't show an aura. An illusion on a high ceiling, or at the end of a long room will require the PC to get close to even tell there is magic involved. Use that to your advantage to lure them into non-magical traps. After a few, they will learn to stay back :)

5. This is also eating up their concentration slot, as the invocation lets them cast the spell without using a slot but doesn't relive them of the need for concentration. So they can't have another concentration spell active. And if something happens that requires them to make a concentration roll and they fail, there could be a brief period of time where an illusion could pop up and the warlock won't be able to sense magic, or determine that it is just an illusion. So you could have a mundane threat that injures the warlock, breaks their concentration, then you trigger an illusion that they do react to. Your warlock may forget to recast the Detect Magic spell or prioritize other actions instead.

6. Mundane creatures with magic items will also trigger the magic sense, but the warlock will have to burn an action to see the source. Have a series of skeletons come at them, the first two just illusions, by the third the warlock may make an assumption that it too is just an illusion and then someone gets hit with the magic sword a real skeleton is carrying! Or vice versa, so the party wastes resources attempting to attack the third illusionary skeleton after fighting the first 2 real ones.

But again, you should embrace this ability, as it is the PCs area to shine and something they obviously care about, since they took that invocation over other ones. Be careful trying to negate it completely. Instead use it to make the character take risks.

Answer 2

Do I have to tell them it's illusion magic?

Yes. Your warlock has cast detect magic, so they can see auras--when they use their action to do so!--and learn schools.¹ Anything less would be like telling your Sharpshooter that they can't ignore half-cover because you designed an encounter with lots of cover in mind, and you don't want them "ruining" it. In either case the player has made a choice to invest in a particular feature, at the cost of other investments they could have made, deserves to use that investment as written.² (See also Can Detect Magic reveal illusions? for some more detail on the mechanics of this interaction.)

Put another way, let's turn it around: what's the list of other spells that you currently don't allow to function as written when other characters cast them?

As for how it works in storytelling, I usually go with "you sense illusory magic around you--all is not as it seems. The wardrobe seems to shimmer before your eyes..." &c. Another common tactic is to say "you sense this room is layered with illusion: what does it look like to you?" I.e. let the players in on a little of the worldbuilding fun/responsibility. For more advice try the following Q&A:

• Creating a city made of illusion that won't be quickly figured out
• How to use Illusions on players properly?
• What would prompt an Intelligence(investigation) check into an illusion?

along with dozens of other questions on 5e illusions.

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^{1 - There's some question of exactly how much they'll see the auras/sense the school, though. It depends on how the illusions are cast. By a plain reading an illusion cast on an object/creature would show aura/radiate school.}

^{An illusion cast "in space," as it were, is much trickier. Some (incl. me) might argue it is necessarily cast on some object, and will aura/radiate. Some others (incl. Jeremy Crawford, via twitter (for the moment)) argue it can be cast in free space and yield no aura/school. But this-all doesn't matter. Assume, ad arguendum, that some of your illusions will have auras, some may not. And read on, because the mechanics of detect magic aren't the real issue here. Here's the issue: your player has a cool toy, and you're contemplating taking it away.}

^{2 - Frankly: you've got a warlock who's chosen something other than agonizing blast, armor of shadows, eldritch spear, or repelling blast. I'd run with it all the way to the bank!}

Answer 3

Not to rehash the wonderful things nitsua60 and JasonK have already posted I will primarily focus on handling a player with this ability, as I am definitely one of them.

Any Illusionist (worth their weight) that creates a "haunted" mansion with a illusions everywhere would also have red herrings... Mixing real spell dangers with illusory as well as fake spells using Nystal's Magic Aura. This sort of situation can have them pursuing an untamed ornithoid or a while, while the ghosts regroup.

Example: a wall with Nystal's shows up as magical and registers as an illusion but they can't figure out why they can't put their hand through it like the last wall they encountered.

On the converse you can also use Nystal's to negate the radiation of magic from an illusion. PHB 263 and has the added benefit of becoming permanent if cast 30 days in a row (Thank you JoshClark for reminding me).

Mask. You change the way the target appears to spells and magical effects that detect creature types, such as a paladin’s Divine Sense or the trigger of a symbol spell. You choose a creature type and other spells and magical effects treat the target as if it were a creature of that type or of that alignment.

Nondetection will also work although it is a bit more expensive on the slots.

Also Detect Magic has the cost of having concentration so if they have another spell they want to keep up they cannot use it, nor in fact can they ready a spell either as that too requires concentration. Opportunity cost.

Answer 4

Detect Magic shows "a faint aura around any visible creature or object in the area that bears magic." However as explained in "Can Detect Magic reveal illusions?", an illusion is not a creature nor an object; the wording of illusion-creating spells consistently refer to "an image of a creature or object." Additionally, the Wizard class feature Illusory Reality makes it clear illusory objects aren't real:

By 14th level, you have learned the secret of weaving shadow magic into your illusions to give them a semi-reality. When you cast an illusion spell of 1st level or higher, you can choose one inanimate, nonmagical object that is part of the illusion and make that object real.

Jeremy Crawford has also confirmed this:

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

So the Warlock can only see an aura if you've cast an illusion on a creature or object to alter its appearance. For illusions that simply create images, the Warlock only knows there's magic within 30 feet, but not its location or school of magic:

Sensing a presence means you sense something is present, not its location. "Wait, magic is nearby..."

However if the party has any magic whatsoever - active spells, the Warlock's Pact Weapon, a Wild Shaped Druid, a scroll, or even just a potion of healing - the Warlock won't realize a new magical effect entered his range because they're already detecting magic. The Warlock would have to remove all spells and magic items from his presence (likely separating from the rest of the party) just to tell they've encountered a magical effect.

The Warlock can use basic geometry and movement to pinpoint the location of a single stationary illusion. However this is time-consuming and becomes difficult if there's multiple magical effects in the area and/or the effects can move. They also run the risk of passing over a trap covered by an illusion, and will be too busy moving in specific directions to check walls and objects for solidity.

In the end, it probably won't cause any significant problems for you. The party won't be able to find the illusions instantly, and the Warlock is giving up one of their invocations and their concentration to do this.