In an attempt to make hand to hand combat feel a tiny bit more realistic, I've set out to craft a house rule that effectively enables Two-Weapon Fighting to work with unarmed. Most hominids instinctively swing their arms in combat whether trying to pummel a victim or disrupt an assailant, and even untrained combatants do this to varying degrees of effectiveness. Thus I want to allow my players that option if they find themselves fighting in close quarters without weapons.

What I don't want to do is accidentally open up that can of exploitable worms that they cleaned up nicely when they made unarmed not count as a weapon. Figured I'd run the text by the community and see of anyone can think of any unintentional ramifications.

I'd modify it by adding the underlined text. Take a look and if you see a loophole or problem, please comment!

Two-Weapon Fighting

When you take the Attack action and attack with a light melee weapon that you’re holding in one hand or make an unarmed strike using one hand, you can use a bonus action to attack with a different light melee weapon that you’re holding in the other hand or make an unarmed strike with the other hand.

You don’t add your ability modifier to the damage of the bonus attack, unless that modifier is negative.

The main can of worms this opens is, really, already open, just not as far: monsters that can make unarmed strikes with both hands empty (or attack with a light weapon with another in the other hand (or some combination of the two)).  It nerfs monks, yeah, but only a tiny bit. 

Excellent points! I did not think about how it affects monks relatively by buffing everyone else. Perhaps I might wrangle it in by applying a penalty to the second attack (e.g. disadvantage or no proficiency bonus).

That makes sense to me as well: while it is instinctive to swing our arms at a threat, it's by no means naturally coordinated. People get tunnel vision and go wild without the discipline of repetitious training (e.g. monks).

Or do you think that might be overkill on penalties since it already lacks the ability damage bonus?

D&D's two-weapon fighting rules aren't very realistic in the first place, so if that's your goal, you should probably start with a different rule. There's not much of a difference in attack speed between swinging one arm repeatedly and alternating your arms, and you generally wouldn't want to swing both weapons at the same time. Two-weapon fighting was mainly used in duels; you've got a rapier and the other person also has a rapier, so if you've got a knife in your left hand that offers a lot of hand protection, you can try to parry with your left hand and simultaneously attack with the right. The way D&D handles TWF is silly; there's not much of a point to the classic rapier-and-dagger combo without the Two-Weapon Fighting feat, and if you do have it, then the best thing for you to do is wield two longswords or rapiers instead.

If you've got a decent length weapon and your opponent also has one, you wouldn't try to punch them under normal circumstances. You'd have to get closer to them to do that, and since they have more reach with their weapon, you're going to have to move through their attack range before you can get close enough to hit them. Even if you get away with it, you're probably not going to incapacitate them instantly with an unarmed strike, so again, you run the risk of getting fatally wounded. There are historical accounts of duels where one side fatally wounded the other with a thrust, but the other guy still hit back right after getting stabbed, so even with weapons that kind of thing was a risk.

So on the one hand, extending the TWF rules to work with unarmed strikes isn't much of a stretch and won't break much, but the TWF rules aren't very realistic in the first place so it's not exactly achieving your goal.

Thanks for the feedback!

I recognize what you mean about Two-Weapon Fighting not standing up to realism, especially combined with the Dual Wielding feat--almost all of my players who go that route end up with the longswords!

A lot of the realism I'll adjust for and "wing it" in the moment via DM's jurisdiction, like allowing spontaneous bonus actions for quick jabs or the like when the circumstances make it sensible. In this case I'm just trying to throw a blanket on something that has come up over and over in my games. Like you said, it's not achieving the goal, but I hope it's a small step that way!

Also a brief, but relevant anecdote to illustrate one of the points you made: our rogue/swashbuckler discovered last night the risky business of trying to close a gap through a weapon's range.

She picked a fight with one of the NPCs (a ranger) who went for her own knife; the rogue rushed the ranger trying to stuff the knife draw and didn't make it on the initiative roll, so the ranger got the knife out just as the rogue got close--and rolled a nat 20 on a quick slash. Cut the rogue's chest and neck really bad and nearly killed her.

My first thought was "what about monks?" But when I thought about it, two-weapon fighting, doesn't add damage modifiers to bonus action attack without the fighting style (unless it is negative), so it is a bonus action attack that deals 1 damage of it hits.

Of course, if characters are really converned about use that bonus action, they can pick up a stick and call it an improvised club. Problem solved, no house rule necessary.

Keep in mind that unarmed attacks aren't just with fists. They also represent kicks, knees, elbows, head buts and so forth.

That said like DxJxC mentions it is only one more potential point of damage without the fighting style, so I don't see much issue in that respect.

Typically speaking a more realistic result for two-weapon fighting in D&D would be a small bonus to the attack roll (say a +1), as second weapons were mostly use defensively, but could take advantage of additional gaps in an opponents defense (essentially making one more likely to hit).  However the second attack is a bit of a tradition in D&D.

Quote from DxJxC >>

My first thought was "what about monks?" But when I thought about it, two-weapon fighting, doesn't add damage modifiers to bonus action attack without the fighting style (unless it is negative), so it is a bonus action attack that deals 1 damage of it hits.

Of course, if characters are really converned about use that bonus action, they can pick up a stick and call it an improvised club. Problem solved, no house rule necessary.

I think you're right about it not being nearly as viable as monks. Only dealing 1 damage before mods will never keep up with the monk's scaling damage output. Seems there's not much need for any penalties.

As far as picking up the stick, you make an excellent point--it's precisely the reason why I want to house rule it. A character can swing two sticks at an opponent via one Attack action, but the moment they drop one of these sticks they can no longer use one of their arms to substitute! Not from prohibitive rules, but because the game seems to provide no way to do it.

Quote from Loswaith >>

Keep in mind that unarmed attacks aren't just with fists. They also represent kicks, knees, elbows, head buts and so forth.

That said like DxJxC mentions it is only one more potential point of damage without the fighting style, so I don't see much issue in that respect.

Typically speaking a more realistic result for two-weapon fighting in D&D would be a small bonus to the attack roll (say a +1), as second weapons were mostly use defensively, but could take advantage of additional gaps in an opponents defense (essentially making one more likely to hit).  However the second attack is a bit of a tradition in D&D.

In this case I initially wanted to limit it to hands only, hence the original phrasing above ("using the other hand") to simulate pummeling. But you make a good point about the default unarmed strike including other body parts used as weapons.

Perhaps the second strike could be an open ended unarmed strike rather than necessitating the other hand?