This is a house rule I thought of a while ago, but I'm not sure how game breaking it is.
Basically, anyone (including an enemy) can use an Action to make a social interaction to consume a creature's reaction. The easiest use of this is to deplete their reaction, so they can't make an opportunity attack against an ally.
Example:
One of my allies is within 5 ft of an enemy. On my turn, I use my action to taunt the enemy. If successful he must use his reaction, if any, to return my taunt.
I'm convinced that based on action economy, a creature must sacrifice one action to eliminate one reaction, so this is an equal trade.
This is a rule made to accommodate a player that likes to use social interaction in combat. We've done social interactions without set rules, and he said it would be fun if the social interactions have some mechanical consequence.
Is this house rule balanced?
Best Answer
This rule is not balanced because its costs are too low.
You're granting the player a cost-free disengage for him and all of his allies.
The aided disengage you describe is strictly better than an ordinary disengage, which costs an action but only works for you, and not your allies. You're not trading an action for a reaction--you're gaining both your enemy's reaction as well as your ally's action. This is an enormous benefit--I can think of dozens of times where I wish I could have traded my action for an ally's action, to let them cast a spell, run away, or do some other important thing with their action.
The fact that doing this costs nothing, class-wise, means that you're giving away a powerful situational ability at zero cost.
Additionally, there is no saving throw, so it works 100% of the time. This runs counter to pretty much everything in the game; only the most powerful of effects, like Power Word Kill, impose their full effect with no save.
If you're really going to implement this rule, I would suggest making it a part of a feat, maybe alongside a +1 to CHA, so that it has some character-building cost, and adding a save or an opposed check, so that it doesn't work all the time.
Compare to published spells
Compelled Duel seems to be somewhat similar to what you're trying to do:
This effect seems like a good use of "social interaction in combat" that is reasonably balanced. Note that it not only costs an action, but it also costs a spell slot and a class feature.
As @T.E.D suggested in a comment on another answer, you can also consider the cantrip Vicious Mockery, which both deals damage and gives disadvantage on a single attack--this is also similar to the proposed ability.
These spells are class-limited, but if you want to introduce "social combat," I would suggest adding these spells to the spell list of whatever class your player is playing, granting these spells via a feat, or simply granting them this spell to use as a magic item or something.