Is it possible to eavesdrop on Rary's Telepathic Bond?

Question

The spell Rary's Telepathic Bond (PHB, 270; the SRD version of the spell is Telepathic Bond) allows targets to communicate telepathically for 1 hour, regardless of common languages or distance.

You forge a telepathic link among up to eight willing creatures of your choice within range, psychically linking each creature to all the others for the duration. Creatures with Intelligence scores of 2 or less aren’t affected by this spell.

Until the spell ends, the targets can communicate telepathically through the bond whether or not they have a common language. The communication is possible over any distance, though it can’t extend to other planes of existence.

Is it possible for someone/something other than the targets to hear this telepathy as well?

As an example, if a mind flayer was near one of the targets, would the illithid be able to hear all of the telepathic conversations, or just the outgoing messages sent by the target? Or none at all?

Answer 1

Elder Brains Can Do This

Elder brains are pretty much the end result of the most powerful illithids, so the ability to do so isn't particularly widespread, but per RAW it is the most concrete example I'm aware of that covers what you're looking for. They are discussed in greater detail in Volo's Guide to Monsters, but one of their Regional Effects is the ability to eavesdrop on any telepathic communications that occur within 5 miles of them.

The elder brain can overhead any telepathic conversations happening within 5 miles of it. The creature that initiated the telepathic conversation makes a DC18 Wisdom (Insight) check when the telepathic contact is first established. If the check succeeds, the creature is aware that something is eavesdropping on the conversation. The nature of the eavesdropper isn't revealed...

In addition, you've the the Detect Thoughts option presented by Ryan, but that infers that eavesdropping is possible and is subject to some DM adjudication. This DM might give you only half the conversation if using that method.

Answer 2

Yes, indirectly, by reading a creature's thoughts

The detect thoughts spell allows the caster to (among other things):

learn the surface thoughts of the [target] creature – what is most on its mind in that moment.

I think it's fair to say that anything a creature is attempting to directly communicate to another creature (whether verbally or telepathically) qualifies as a surface thought and/or what is most on that creature's mind. Similarly, any communication the creature receives via the telepathic bond would likely be foremost in their thoughts at the moment it is received, unless they are very distracted (e.g. in combat), or they know that someone is attempting to read their thoughts and are making a concerted effort to avoid thinking about it.

However, one important caveat to this way of indirectly spying on the telepathic bond by reading thoughts is that you don't automatically know which thoughts are telepathic communication and which thoughts are just thoughts the creature is thinking to themselves. In most cases, it probably won't be too difficult to infer, but it's something to be aware of regardless.

Also note that Rary's telepathic bond can link more than two people. If Alice, Bob, and Claire are linked via this spell, and Alice sends a thought specifically to Bob, you probably can't read that thought in Claire's mind, unless the DM rules that the any communication through the spell is sent to all targets.

In short, I don't think there's a way eavesdrop directly on the telepathic bond itself, but there are plenty of ways to eavesdrop on the minds of those who share the telepathic bond, and this can allow you to indirectly infer what information was shared. Keep in mind that detect thoughts is only one of many ways to read minds, and each mind-reading ability is a bit different. Some more limited mind-reading abilities may not give the necessary information.