I bought [Dropping Below Zero: A Chilling Adventure on the Road to Icewind Dale](https://www.dmsguild.com/product/321754/Dropping-Below-Zero-A-Chilling-Adventure-on-the-Road-to-Icewind-Dale) from DMs Guild. I like the adventure a lot -- I think it's worth the money and that people should purchase it.

This adventure has a friendly (or not unfriendly) monster that has an action it can take an unlimited number of times. Essentially, at will it can suppress the strong emotions of creatures around it. Affected creatures act as if they're charmed against all enemies and dangers, for 1 minute. There's a reasonable DC and affected creatures can do a saving throw each turn, and creatures can be  only be affected by this effect once every 24 hours.

I think it's a cool action (it affects all creatures, not just enemies) and a cool creature, and I'm considering allowing my party to take it on as a sidekick if they choose -- the creature is documented as having CR 1/2. My concern is that this capability might be a bit OP. Anything immune to charm won't be affected by this effect, but a lot of things will. I'm worried it'll throw off the balance of the game (this will be a 1st or 2nd level party) -- either fights will be too easy, and maybe the accessibility of this strong and obvious strategy will make fights repetitive.

I'm looking for advice from experienced DMs. 

Can he control which creatures are affected, or is it everyone nearby? If not, then the party will be charmed as well and cannot attack the enemies. The enemies might pass their save and have no such restrictions.

Charmed conditions generally end if the charmer attacks the charmed, so his ability might buy them one surprise round, but after that it'll be a free for all again. If you want to ensure he doesn't impact the game too much, fudge his intitative to go after the enemies and before the PCs. That way, monsters attack, then they get charmed, then PCs attack and break the charm, and repeat - making it do, effectively, nothing. 

He can't control who gets charmed. But I'm hoping my players are smart enough to realize they can hack this situation, e.g. by staying 30ft away.

Great idea about initiative.

I might just see how it goes. I can always later tweak things -- maybe the sidekick decides to leave for some reason...  Or maybe he's a temperamental beast and doesn't feel like playing optimally for the players.

If he has to stay 30 feet away then chances are he'll die fairly quick. He enters a room by himself, the party are more than a full turn of movement away. The charm effect is resisted and everyone concentrates attacks on the only target they see. 

Good point. So this effect is perhaps not overpowered then, since it doesn't favor friend over foe. It might get annoying, however... at which point the PCs are welcome to get rid of the sidekick somehow.

Its uses are probably much more in the region of "crap, this fight's too dangerous - get him to charm everyone so we can run away without them pursuing!"