Is this house-rule limiting Disengage broken?

Question

I'm a new DM and I recently ruled in a game that a PC that disengages takes no opportunity attacks from enemies they are currently in melee range of, but will take opportunity attacks if they try to move through a space within melee range of other enemies.

In other words, the Disengage action will only prevent opportunity attacks by enemies within melee range at the time the action is taken.

This is incorrect according to the rules, but I intend to make it a permanent house-rule. My reasoning is that it makes no sense to me that backing away from enemies in front of you somehow makes you immune to all other opportunity attacks regardless of direction.

I think I can get the players onboard, and the same rule will apply to NPCs and monsters (of course) - but is this house-rule broken, mechanics-wise?

I'm defining "broken" here as giving an unfair combat advantage to certain races, classes, or monsters.

Answer 1

I wouldn't do it, because Cost Matters

The key component that this idea is missing is the cost vs value of the Disengage action. It is an action, a special ability provided by a class feature or a special ability provided by a magical effect.

But the universal cost of the Disengage is to gain it by spending an action. Everything is (mostly) balanced around this cost. This means that Disengage is intended to be, at least somewhat, as powerful as other uses of your action, such as:

• Attacking
• Casting a spell
• Hiding

With this change, you are making Disengage much much worse. And you can do that. But you shouldn't until you address the existing cost of it. If you don't want it to be as valuable as the Attack action, change the necessary cost of the Disengage so it's not as expensive as an Attack.

My suggestion? Half your movement to Disengage, or do the ol' 5 foot step rules from prior editions.

Your change also hinders casters and rogues quite a bit, as they have few defensive tools if they are surrounded, and one of their tools is the Disengage action via special abilities. Your melee fighters will hardly use it at all, so expect there to be a power difference between melee/ranged characters, favoring melee, in certain fights.

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Overall, though, my suggestion is to not bother with it. There is a strong lack of mobility with players in DnD 5e, due to not needing to flank, few AoE abilities that center on the caster, and weak opportunity attack uses, so melee combat is generally a stagnant mosh pit of damage. If you impact Disengage in this way, combat will move more in the direction that prevents mobility.

On the flip side, melee lines would be harder to break, and units in protected positions would have an advantage.

Answer 2

There is another way how to solve this

Any additional house rule might have its complications. Players didn't read about it in PHB, published adventures don't take it into account, it wasn't playtested, et cetera. Before introducing a new house rule, always ask these three questions:

1. What problem are you trying to solve using this new rule?
2. Can this problem be solved using existing rules?
3. If you introduce a new rule, will it cause any game-braking changes?

You ask us about 3, but you completely ignore 1 and 2.

What problem are you trying to solve? The only reasoning was:

it makes no sense to me that backing away from enemies in front of you somehow makes you immune to all other opportunity attacks regardless of direction

If all your players are okay with the default version of Disengage, and the only problem is that "it makes no sense" to you, that means it is not a rules problem. This problem shouldn't be solved by modifying the rules. It can be solved by finding a convincing in-world explanation of Disengage mechanics for you and your players.

Your confusion comes from a couple of misconceptions. Disengage Action is not "backing away", and it is not about direction a creature face. First and foremost, Disengage Action is game mechanics; you, the DM, use these mechanics to adjudicate character's actions. A character triggers a OA when they "past their foes" and "moving heedlessly". If he/she makes efforts not to be "moving heedlessly", we call these efforts Disengage:

In a fight, everyone is constantly watching for enemies to drop their guard. You can rarely move heedlessly past your foes without putting yourself in danger; doing so provokes an opportunity attack

Rules are DM's tools. By limiting Disengage you take a useful tool away from yourself.

Imagine your player announcing:

I carefully walk past these fighting group, avoiding any risks of being hit in the process

How would you adjudicate this? With default Disengage mechanics you can just use them, saying "Okay, but you have no time to do anything more this turn". With crippled version of Disengage the only remaining tool would be Dodge, which barely complies with the player's intent.

Answer 3

Combat in general will not be much different

Your house-rule makes the Disengage action much worse, but because this action is not used much, the effect on the overall combat will be limited. In other words: most combats are very stationary, so an incentive to move less simply won't have a big impact on the game.

When someone has to move past multiple creatures no matter what, they will just use the Dodge action instead of the Disengage action.

The only big exception are:

Creatures that can Disengage as a bonus action

These creatures include NPCs such as goblins and playable characters such as Rogues and (to a lesser extent Monks). These creatures are designed to easily avoid all opportunity attacks. Disengaging as a bonus action is a core defensive and offensive feature which your house rule weakens significantly. As such I think that your house-rule should not apply to bonus actions.

Your house-rule does not solve the problem you described

Problem:

it makes no sense to me that backing away from enemies in front of you somehow makes you immune to all other opportunity attacks regardless of direction.

Solution(?):

the Disengage action will only prevent opportunity attacks by enemies within melee range at the time the action is taken.

Your house-rule basically narrows the 'range' of the Disengage action, instead of the 'direction'. Notice how it does nothing to solve your problem as you've described it.

Usually , D&D 5e does not track which way creatures face, so stuff like "in front of you" has little to no meaning. However, there is an Optional Rule for Facing in the DMG (p252), into which you could integrate a Disengage house-rule that actually solves your problem.

Your house-rule is based on a wrong assumption

You assume that disengaging means "backing away from enemies in front of you", but that is simply not the case. The Disengage action makes no sense to you because of this incorrect assumption.

The Disengage action says:

If you take the Disengage action, your movement doesn't provoke opportunity attacks for the rest of the turn.

As you can see, nowhere does the disengage action mention a direction. When you disengage, you aren't merely "backing away from enemies in front of you", you are doing your very best to move without exploitable openings for the rest of the turn.

TL;DR

Overall, I do not think this house-rule is worth the trouble because it won't change the game much at all, besides nerfing Rogues. Moreover, I question whether you can get your players on board with a house-rule that does not solve the intended problem and that is based on an incorrect assumption.

Answer 4

This will probably be fine, but it's a big change to combat. Don't do it if you want more "realism", because D&D combat is super-abstracted anyway, with its 6-second rounds, lack of of facing, and completely-divorced-from-any-direct-model armor class and hit points. Instead, do it if you want a game with more emphasis on grid tactics in combat.

It was basically the rule in 3.5E — see attacks of opportunity and the withdraw action, which only makes that first square safe. Having played both editions, the main consequence of this and the other simplifications of position-related things in 5E is that you need to keep track of space, position, and movement much more carefully, and everyone needs to spend more effort thinking about tactics. You definitely want to use this with miniatures on a grid. You might also want to import 3.5E's five-foot step rule (see the link above) which allows some jostling for position.

3.5E also has feature chains (feats and class features) that let characters focused on mobility avoid these attacks. I think that reasonably corresponds Ruse's very valid point that doing this in 5E would make monks and rogues weaker. To counteract that, rather than importing 3.5's options wholesale (because that'd be a lot of game tweaking) instead beef up the monk and rogue Disengage features to be more powerful than the standard action (and possibly allow something along those lines as a fighter option). That way, classes where movement is their thing aren't hobbled but you can still get more tactical positioning and the general trope of melee fighters keeping opponents engaged.

In fact, if you really like this, you may want to look more at the combat rules for 4E. In that game, designers effectively decided that position on a grid is a first-class tool in the combat simulation, right up there with the HP and AC abstractions. Many powers and features are directly concerned with position. If you have a group who really enjoys combat tactics, you probably should start with 5E's optional facing rules and the "mark" option, but you may consider house rules that go even further. This will change the balance of the game, but not necessarily in a way that's "broken" or "unfair" — at least not if everyone is bought-in and understands.