[RPG] How should I prevent a player from instantly recognizing a magical impostor without making them feel cheated


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.

Best Answer

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