Is this homebrew arcane communication device abusable?

Question

My group will be performing some undercover scouting in restricted territory that will limit participants to just a few members of the group. I don't want to force a complete party split, however, so I came up with the idea of letting them borrow (and potentially keep if they do well) an FBI-like earpiece item that could give those who are left behind a chance at backseat "helping". It's basically one ear piece, and one speaker. I followed the style of Sending Stones for activation and mechanics.

Is this homebrew magic item abusable? How could I regret giving this out?

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Resonant Tether

Wondrous item, Minor, Rare

Description:
The Resonant Tether has two parts: A hard ball of wax tiny enough to squeeze into someone's ear and a thin 1 foot wide metal drum covered by a hard waxy head. The ball is strangely weighted, and far heavier than it looks. The drum is useless for music, as any tap or strike on the head creates no sound what-so-ever.

Features:
While you touch one of the parts, you can use an action to silently cast Clairvoyance from it. The spell uses hearing as it's sensor option, and the sensor target is attached to the opposite part. If a creature is touching the opposing part, it can use its reaction to silently cast Clairvoyance in a similar manner from it. If no creature bears the opposite part or if no creature uses their reaction for the associated casting, you know that fact as soon as you use the Resonant Tether and don't cast the spell. If one bearer loses concentration on the Clairvoyance spell, both spells are ended.

The sound transmitted by the spell emits from its spell source object barely loud enough to be audible within a limited range. The waxy ball's range is a 1ft radius, and the metal drum's range is a 10ft radius. The transmitted sound is inaudible beyond this range.

At the end of each use of the Resonant Tether, roll a d4 and add the number of times it has been used since dawn. If the total is 6 or higher, the Resonant Tether releases a squeal of high pitched arcane feedback. Any creature within the associated range of either object must make a DC 13 constitution saving throw or become stunned for 1 round and deafened for 1 minute. In addition, the squeal is audible (but not harmful) to all creatures within 10 times the associated range.

Notes:

• This (hopefully) acts as one end being an ear piece, and the other end being a speaker.
• The Clairvoyance spell has a duration of 10 minutes, range of 1 mile and requires concentration.
• The Clairvoyance spell normally has verbal component and a casting time of 10 minutes, but this is bypassed by "use an action to silently cast" and "use its reaction to silently cast".

Answer 1

Any magic item can be abused in certain scenarios.

What you describe here is not game breaking, it really only allows the party to communicate. If you don't want them to rely on it to heavily (because they probably will if they permanently have it), just put charges on it. Make it only usable X number of times then the magic item doesn't not work.

There are two ways you can do this:

1. Give it a number of charges per day
2. Give it a total number of uses before it permanently stops working

Personally, I like option 2 because it keeps the party on its toes and they cannot rely on it forever. If I was given something that was only usable 3 times, I would be very careful about using it for the proper situation.

Whether or not this can be abused really comes down to your adventure. I honestly can't think of any situation I have personally run where this item would give the PCs any real advantage. Usually when possible parties tend to stick together, so most of the time they can communicate with one and other.

If your future plans would suffer from them having this item full time, go with a charge system. If you do go with a charge based system, you will need to tweak the last paragraph a bit. Personally, I would remove that bit altogether.

Communication between the party is a good thing overall, so why have a potential punishment for using it?

Answer 2

At the end of each use of the Resonant Tether, roll a d4 and add the number of times it has been used since dawn. If the total is 6 or higher, the Resonant Tether releases a squeal of high pitched arcane feedback. Any creature within the associated range of either object must make a DC 13 constitution saving throw or become stunned for 1 round and deafened for 1 minute. In addition, the squeal is audible (but not harmful) to all creatures within 10 times the associated range.

Possible "High Pitched Squeal" Abuse

I would recommend having the device become nonfunctional for (X) minutes after emitting the squeal - otherwise a group of entreprising players might use it as a feedback bomb, triggering it consecutively as a 10ft ranged AoE stun.

Answer 3

Balance is the wrong consideration

I am not smart enough on 5e to give you a useful opinion there, but I have seen the same issue play out in enough other systems to know that balance is the wrong lens. You’re looking at a change in table dynamics, which will overwhelm any balance considerations. I feel it is usually a net positive change, but that depends heavily on what kind of game you already run and what you want to run.

The best analogy for this is (oddly) Call of Cthulhu. CoC has rules for many different eras: Cthulhu Invictus is classic Rome, there’s another for Dark Ages, there’s a Cthulhu By Gaslight, the default 1920s-30s setting, an early ‘90s setting, a modern setting, etc. Nearly all of these play the same. Sure, sometimes you have guns and dynamite, but overall, the feel and atmosphere are very similar. The one exception is Modern, and the reason is cell phones. Instant communication is a literal game changer.

That’s not a bad thing! There is nothing bad about a game where you can call everyone and pass on the key clue you just learned, or call for help, or whatever. It also has some strong metagame-y upsides, in that player knowledge is much more likely to match character knowledge, and everyone can participate and engage in what would otherwise be solo scenes. But it is a very significant tone and metagame change, and you should make a conscious decision that you want that change.

Your specific implementation should follow that decision. If you want this to happen, remove the penalty. If you don’t want it to happen, just directly ban it. Allowing a crippled version is the worst of both worlds (and incidentally is a larger surface for exploits, as other answers have pointed out).

I also suggest that you don’t decide this by yourself. If you’re looking at this (or any other tone change), decide as a group which way you want to go. Most groups I have played with or run for, I would say “go for it!” But I also enjoy playing pre-cell phone Cthulhu, and specifically avoid communications devices there, so it really depends on your group’s preferences for this game.