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I am running a D&D 5e campaign, and we had an interesting case come up in play last night.

The party cast minor illusion to conceal themselves in a cave at the end of a narrow canyon, and then a Spined Devil, with telepathy, spoke to them telepathically (and they replied).

Does the presence of the illusion mean that the Spined Devil could not have known telepathically that they were there?

In other words, since the party had been conversing while within the Spined Devil's telepathic range, even though he was out of sight and ear shot, would there be any reason to think that he would not be able to ascertain their (general) location despite the illusion, once he arrived at that location?

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    \$\begingroup\$ Hi, welcome to RPG.SE! Take the tour to find out how things work here, and visit the help center for more guidance. Thanks for adding the [dnd-5e] tag for us, and don't mind me putting italics on minor illusion, that's not a hard-and-fast rule to follow, it's just something we like to do here to make it match the source book's formatting (RPG.SE is quite an edit-happy culture). Hope you get a good answer soon! \$\endgroup\$
    – NathanS
    Commented Jul 7, 2020 at 15:53

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The spined devil would know the characters are within 120ft but not their specific location

First, I have to say that hiding an entire party behind a minor illusion is slightly unlikely since this can only create an object no larger than a 5ft cube. That would likely be quite a squeeze!

Nevertheless...

The Telepathy ability is listed as a special "language" in the Monster Manual. There it states:

A telepathic monster doesn’t need to see a contacted creature and can end the telepathic contact at any time

Also:

A creature without telepathy can receive and respond to telepathic messages but can’t initiate or terminate a telepathic conversation.

So there is no reason to believe that the devil couldn't attempt to speak telepathically a party member if it suspected they were in range. And that person could even respond. But there is nothing in this general telepathy ability that indicates that the creature can use it to locate someone (if it did, it would say so).

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  • \$\begingroup\$ An interesting thing would be if the telepathy works as sound for us, that we can detect the strength of the sound when or if we can hear it. I mean, imagine that at 120ft you hear the quite clearly and, at 120.1ft it suddenly stops. Obviously that is a DM job to determine. But that thought amuse me. \$\endgroup\$
    – Chepelink
    Commented Jul 7, 2020 at 20:46
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Telepathy doesn’t tell you much about where the person you are communicating with is

The rules for Telepathy, at least as far as it applies to communications available to monsters, are laid out in the Monster Manual (under Languages, page 9 in my version).

The important bits here are (paraphrased):

  • The creature target with Telepathy needs to be in range
  • If the contacted creature ceases to be in range the communication link breaks
  • The contacted creature doesn’t need to share a language with the Telepath
  • The Telepath does not need to be able to see the contacted creature

In particular, notable by their absence:

  • Nowhere is it specified that Telepath needs to know the location of the creature they’re contacting
  • Nowhere is it specified that Telepaths learn the location of the creature they’re contacting
  • Nowhere is it specified that there needs to be a direct, unobstructed path from the Telepath to the contacted creature.
  • Nowhere is it specified that the Telepath knows if the message it was sending is indeed received

But the Bone Devil might find the party in this particular situation anyway

So about the only useful thing the Bone Devil can learn is if the creature it is communicating with is in range (120 feet for this creature), and even that is only the case if the contacted creature replies. As long as they keep talking, a Bone Devil (an intelligent creature) might use this knowledge to attempt to pinpoint the location of the creature it is talking to by seeing where the contact breaks. If the Bone Devil has an excellent spatial sense and a grasp on basic trigonometry they will be able to locate the party this way by finding three¹ points where the contact breaks (one already gives them a general direction to look in, and two narrows the location down to two possible points one of which may well be implausible). They won’t automatically see through the illusion, but given that they know now where the party is they may have cause to doubt it and investigate further, and Minor Illusion doesn’t hold up well under scrutiny.

So in that sense it certainly isn’t a great idea to keep talking to someone you are trying to hide from, even if it doesn’t immediately give your position away.

This also provides an interesting RP opportunity where the devil might goad the party into replying, with the contact mysteriously breaking for fractions of a second, and finally culminating in the Bone Devil confidently approaching the parties hiding spot. It’s the sort of trick that really only works once though.

1: Technically speaking, D&D takes place in a 3D world so we’d really need to find a unique sphere rather than a circle which requires knowing an additional point. However, given that most adventures take place on relatively level ground this doesn’t really come up much in practice. Certainly something to keep in mind when you’re hiding from the devil on top of a tree or something though.

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    \$\begingroup\$ Excellent answer, but if the Spine Devil assumes invisibility (rather than statically hiding with an illusion), it becomes much harder to pinpoint a potentially moving target. \$\endgroup\$
    – raithyn
    Commented Jul 7, 2020 at 16:41
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    \$\begingroup\$ @raithyn Indeed, although my mind would first go to hiding than to invisibility. Granted, I am also not a Bone Devil. \$\endgroup\$
    – Cubic
    Commented Jul 7, 2020 at 16:52
  • \$\begingroup\$ This is incredibly helpful! Another factor that was relevant to the encounter is that they had recently passed through the canyon, and there had been recent combat, which would have left numerous signs. My reasoning was that he would have reached out telepathically for several reasons: 1) They were whispering after he entered a 120 ft radius 2) There were signs of combat and 3) He travels the road regularly and is quite familiar with it. Once one of the characters behind the illusion (it was at the entrance to the cave) replied to the telepathy, the Devil know they were in the cave. \$\endgroup\$
    – BprDM
    Commented Jul 7, 2020 at 17:25

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