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In the campaign I'm running, I'm looking to introduce an NPC who is secretly an antagonistic oni in disguise. An oni's shapechange feature, however, is explicitly described as magical, and one of my players is a warlock with the Eldritch Sight invocation. I'm worrying that the warlock might decide to use Eldritch Sight during the first encounter with the oni, before the party should have any reason to suspect them, and expose the NPC as an impostor way before I'd like them to.

However, I don't like to just decide that the oni is immune to the effects of Detect Magic, or has a magic item that protects their identity, or knows Nystul's Magic Aura, or anything like that. I feel like that wouldn't be fair to the warlock's player; after all, they specifically took the invocation to be able to detect magic at their leisure.

Ideally, I'd like to find a way to hint that the NPC might have some mystery surrounding them if the warlock decides to have ES active, but subtly enough to where the PCs won't immediately distrust or attack the NPC.

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RAW, this isn't immediately an issue

As you mentioned in your question, a Warlock's Eldritch Sight lets you Detect Magic at will.

The spell description for Detect Magic states:

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.

The Warlock only immediately knows that your NPC bears Transmutation magic. That's mysterious, but not inherently alarming.

There's a pretty extensive list of Transmutation spells. Many can immediately be ruled out, such as those with particularly short durations, obvious tells (eg, Barkskin), but that still leaves a laundry list of possibilities outside of Shapechange, True Polymorph, etc. Since many spells have targets other than "Self", it's not even a given that the NPC can cast magic themself.

Reward the knowledge

Your Warlock invested in this invocation, so you have several options that could play to various player, roleplaying, and character strengths.

  • Encourage sleuthing (Sleight of Hand to search their belongings), overt research (Investigation), or covert interrogation (Cha skills) during downtime activity
  • If your players are good at separating player/character agency, carefully parcel out information that will encourage in-character discussions
  • If not, give slightly more information to the Warlock's player than the others. This could create some minor (healthy) party tension but, again, encourages roleplaying
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    \$\begingroup\$ Since the Oni is likely aware of the way detect magic works he might activly seek to have some of the transmutation spells with visible effects going. - Ie. yes the NPC has an aura of transformation magic - but that is hardly surprising given that his skin is bark. \$\endgroup\$
    – Taemyr
    Feb 4 at 11:56
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    \$\begingroup\$ @Taemyr: I'll note that this is actually ideal. Giving the players clues that can be explained away by the NPC means that the player (hopefully) won't recognize the magical imposter straight away, but won't feel cheated once they do. \$\endgroup\$
    – Brian
    Feb 4 at 14:42
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    \$\begingroup\$ A quick look through the least, have the meeting somewhere dark and the oni under the effect of darkvision. \$\endgroup\$ Feb 4 at 17:41
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Oni are pretty clever, so I'm sure this one gave it some thought. When you meddle in the affairs of powerful folk, you're bound to end up on the wrong side of Detect Magic at some point, so you need to have a story ready.

Fortunately, Detect Magic only tells you that there's "a magical effect" and what school it is. That means that your Oni just needs to have a decent-ish reason to be affected by a spell from the Transmutation school.

If I were an Oni, taking on a long-term magical shape, I'd almost certainly pick one that presents as a spell-caster of some sort. That way you can just BS your way out of suspicion with "of course I'm under the effects of magical spells, they're useful".

Then, if pressured, I'd back it up with (for example) "Look, I'm using Enhance Ability because I was organizing my lab" and then lift a big rock or whatever. Even when presenting as a frail looking Wizard, an Oni has their natural 19 Strength, so it should be pretty easy to convince people. Other easy bluffs might be "It's dark out, so I'm using Darkvision" (again, Oni have this naturally, so they can back it up easy), or "I cast Fly to get here faster".

You could even do a nice double-bluff and say "Okay, okay, you got me. I don't really look like this. I shapechanged to look prettier/avoid an annoying ex/because I have some trouble with the local guards."

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    \$\begingroup\$ Or alter self. "I'm self conscious about my appearance... I tend to make my ears smaller, or my nose more shapely" This is a great way to handle the awkward "I'm enchanted, please stop drawing attention to it!" \$\endgroup\$
    – Carson
    Feb 3 at 22:16
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    \$\begingroup\$ Added suggestion: pre-roll the oni's bluff check (so that you can do it without rolling in front of the PCs) and roleplay accordingly when he makes his excuses. \$\endgroup\$
    – Ben Barden
    Feb 3 at 22:28
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    \$\begingroup\$ Or the oni can lie to the party about being "cursed" with some Transmutation effect or something similar. \$\endgroup\$
    – Upper_Case
    Feb 3 at 22:29
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    \$\begingroup\$ This is a reasonable in-game explanation, but the players may very well still be suspicious. Things in fiction tends to exist to serve the plot in some way, and less so as just general world-building or something that happens because the world doesn't revolve around the main characters. If you want players to be open to the latter, you should probably make a habit of it: not only having them run into other wizards that are under the effect of a spell for no plot reason, but also just generally some things happening that don't really concern the players. \$\endgroup\$
    – NotThatGuy
    Feb 4 at 12:54
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    \$\begingroup\$ "An oni cast a curse on me to look like him, so I'd know what it would be like to be hated on sight. It worked really well and I was hated on sight. So I use Alter Self at all times." \$\endgroup\$ Feb 4 at 23:23
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Another option: the Warlock does recognize the oni, and the plot thickens...

The other answers cover the most direct ways to answer your question that occur to me, but I wanted to suggest another approach which I've used. It depends on extra plot setup and requires a bit of experience on the player's part, but the idea is that the Warlock does get the knowledge via Eldritch Sight, but there are plot and/or character reasons that they can't (or shouldn't want to) reveal the truth.

My go-to rationale for that sort of thing is some form of blackmail: a threat the someone can hold over the Warlock's head. If you want or need to maintain the mystery for the Warlock as well, you can obfuscate the oni's identity as the blackmailer and use any of the excellent I'm-transmuted-for-totally-reasonable-reasons approaches in other answers. Or you can have someone else force the secret (patrons are great for this task).

This is my favorite way to ensure a mystery without railroading players, forcing non-RAW changes like convenient immunities, or fudging rolls. It relies on one of my favorite techniques: secret information which the player(s) become(s) complicit in hiding. It not only gives the Warlock's feature choice a meaningful opportunity to shine, but it also gives them a special role in the party and a strong connection between their character and the plot.

By pulling the Warlock into the secret, and/or adding extra content for them to investigate with a tantalizing clue, you take a lot of pressure off of yourself to give perfectly-calibrated clues which entice but do not reveal. And even if the Warlock, as the only PC with the abilities that could pierce the secret before you're ready, does catch on they have an organic motivation to assist you in keeping the secret anyways!

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    \$\begingroup\$ The issue with this one is... well, the warlock has chosen this specific ability, because he wants to be awesome in situations like this one. It should have some payoff. Using DM fiat to ignore his power is bad. Adjusting plot so that he's worse off than he would have been without the power is worse. \$\endgroup\$
    – Ben Barden
    Feb 4 at 1:06
  • \$\begingroup\$ @BenBarden There is some variation based on how well the DM prepares and executes the story, but my experience has been that players enjoy their characters having the spotlight-- there isn't a mechanical- or reward based- penalty, just extra plot elements that let them play a bit more in a situation the other players don't get access to. I disagree that this is ignoring the power (the DM seems set on not allowing the power to obviate the encounter anyhow), but totally appreciate that it may not work for all DMs, players, or tables. \$\endgroup\$
    – Upper_Case
    Feb 4 at 2:52
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Let Your Players Know Something Is Being Hidden

I agree with the existing answers that in general the best course is to let the player know that the polymorphed oni has a transmutation aura but not reveal its true form.

Having said that, there are times it's important to the feel of the game to hide a specific character or object from Eldritch Sight or other auto-win investigation powers. There's two rules of thumb I use for this:

  • Let the players know something is being hidden.
  • Don't overuse these abilities. (Nystul's magic aura, nondetection, etc.)

An Example from Play

In my current campaign there exists an illegal wizard academy dedicated to raising members of a persecuted religious minority. Nystul's magic aura is all over the place. This includes a heavy fog that surround the island and prevents scrying of anything within it. When my players first sailed into the fog I described it in a way that was clearly not natural and allowed arcana checks to reveal that it was magical. One of my players immediately cast identify (then clarified she meant as a ritual of course, so immediately plus 10 minutes). This revealed nothing. No magic, no spells, nada.

Did I bend identify a little for her to cast it on fog? Potentially. Was I counting on her to do that? Certainly. Now the party knows that something here is obscuring their ability to discover specifics of magical properties and they acted accordingly throughout the adventure.

I only have one location in the world where this fog exists and normally allow her to use identify to freely discover magical properties and take the spotlight in explaining them to the party. In this case though, for everyone to enjoy the mystery of the island I had to cut her off from that ability so I set up a way for her to have the spotlight for a scene as she discovered that. From that point on, the other players got to take turns in the spotlight as everyone investigated the island's secrets.

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Let's assume for a moment that the oni is smart and realizes that someone may pick up on the magic operant on their person and even the type (transmutation).

If they spot the Warlock giving them the once over with some intensity (or if they show some startlement when they pickup magic and possibly transmutation), have the oni activate his contingency plan: To give the Warlock a sharp glance, make a kind of surreptitious shushing motion, and look around to see if anyone else saw.

If not, have him wave over the Warlock for a quiet conversation away from others (but not away from sight so as to not seem like am ambush).

If the Warlock cautiously goes to speak to the shifted oni, the oni needs to have a story of why he is shifted, that he is an agent of X (where X is some third party or even someone roundabout back to the party's patrons), and that he's undercover. He can note that the Warlock has perception and awareness, but he could ask the Warlock not to blow his cover or much important work would be lost and an important mission would be blown.

If the Warlock pushes, find a way to make him think he cannot be fully read-in, but he can help. And ask him to keep his discovery from the others in his team because they may not be as good at hiding their reactions as he is or they may let slip something accidentally that could scotch the oni's important (faux) mission.

If the oni can get the Warlock interested in being involved, he can perhaps use the Warlock or at the very least send him off on a key red herring at the right moment.

This doesn't negate the Warlocks ability nor does it stick him with any particular new information that constrains his actions. This simply shows the oni has a second layer of cover to use in the event someone notes his magical aura. The Oni can go on the offensive and try to convince the Warlock to not share information with his team and to help the Oni (on his obviously necessary and noble) mission (false as it may be....).

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Add another layer

Have the oni shapechange, and then have him wear an additional physical layer of makeup/costume.

This should be weird enough to let everyone understand something is odd, but not exactly what nor why.

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Give the Oni one of these amulets.

Amulet of Proof against Detection and Location

While wearing this amulet, you are hidden from divination magic. You can't be targeted by such magic or perceived through magical scrying sensors.

Since it blocks divination magic, detect magic simply won't work on the wearer.

If you don't like the idea of protecting against that specifically, go in the other direction and use "hide in plain sight" methodology. Give the oni a bunch of different magical items and make it obvious they are wearing such. They will expect to see magical effects on him in that case. Keep in mind, they may decide to murder him for his items if they are so inclined.

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    \$\begingroup\$ This seems to go against this part of the question: "However, I don't like to just decide that the oni is immune to the effects of Detect Magic, or has a magic item that protects their identity, or knows Nystul's Magic Aura, or anything like that." \$\endgroup\$
    – Erik
    Feb 4 at 9:12
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    \$\begingroup\$ D&D has lots of "your magic doesn't work" things. They are all bad. Don't use them. \$\endgroup\$ Feb 4 at 18:08

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