Combat in D&D is an abstract affair. It has been this way since day one. And, probably since day two, there were those who preferred a more concrete, blow-by-blow, experience, and came up with various house rules for making combat more detailed. This isn’t just a figure of speech; the Perrin Conventions, for example, suggested 10-second rounds and rules for knockback and knockdown in 1978.
The exact nature of combat varied through editions and versions; the Rules Cyclopedia has many combat maneuvers, while B/X has little to none, making the Fighter significantly weaker. Modern editions of D&D have a tendency of burying maneuvers in feats and powers; even if the maneuver is available to everyone, those without the proper traits would find it sub-optimal or useless.
Fourth edition, for example, had no rules for disarming outside of specific powers. On the other hand, it had a single page dedicated to actions not covered by the rules – the much praised (but not universally loved) page 42 of the DMG.
Fifth edition has a bit of both. It has some general rules for disarming, tripping, shoving, but also some specific classes and feats dealing with combat maneuvers. It has no direct equivalent for page 42, but enough specific rules that can be easily converted to a basic “universal maneuver system”, as we will see.
The Universal Combat stunt rule
This rule is extrapolated from the ones in the PHB and the DMG, as suggested by page 195 of the PHB.
When you want to perform a physical combat stunt (tripping, disarming, shoving etc.), make Strength (Athletics) check OR attack roll against the target’s Strength (Athletics) or Dexterity (Acrobatics) – target’s choice. The target cannot be more than one size larger. If you succeed, the target suffers some minor effect: suffers the “grappled” condition, gets knocked prone, pushed 5 feet away, disarmed, dismounted, blinded, etc. If you have multiple attacks, this counts as a single attack.
The distinction between “Strength (Athletics) check OR attack roll” isn’t very important in 5e, it just means that Dexterity-based attackers will be limited in some maneuvers; disarming is fair game, but grappling will require strength.
Example: A Fighter in the arena, unconcerned with honor, decides to throw sand in his adversary’s eyes. He makes an attack roll against the target’s Strength (Athletics) or Dexterity (acrobatics) and, if it succeeds, the target is blinded until the start of its next turn. The GM decides the trick will work only once per combat (unless the adversary is a Beholder, maybe?).
Recovering from stunts
The target should be able to recover from most stunts as easily as getting up from being prone. This means most conditions inflicted upon the target (blinded, frightened, etc.) should go away with little effort by the beginning of the target’s next turn (or the end, if it’s something that ONLY affects the target in its own turn).
The same reasoning applies to disarming. The way I see, kicking the targets weapon a few feet away is a free action (object interaction), but the target should still be able to get the weapon by moving 10 feet or so (but read “overpowered maneuvers”, below).
Since the stunts described above require a single attack, I would allow fighters with multiple attacks to perform stronger versions of those; for example, spend two attacks and push the target away 10 feet, or use two attacks to make the target prone and disarmed. I would also allow for an “all or nothing roll”, i.e., spend two attacks but roll only once, success means pushing the target 10 feet away – unless the player is trying to game the system in some manner (try to use all attacks at once and spend inspiration, etc.).
Some of my favorite games make weapon choice really relevant. Fifth Edition, to some extent, uses feat to do this. Since we are making a universal rule for combat stunts, I would always take weapons into consideration when deciding if a stunt is possible or not.
As always, common sense (as determined by the tone of the campaign) must be used: you won’t push someone back 5 feet with an arrow in my games, but it might work on yours.
In fact, I even go as far as adding specific combat effects for different weapons in my games, but I realize this isn’t for everybody; it breaks some of 5e’s elegance and oversteps into feat and class territory. Some examples are giving +2 damage for a spear when charging (or set against a charge), adding a +2 bonus to disarm or trip for certain weapons, flails ignore shields, etc.
While writing this rules, I assumed Fifth Edition is a balanced game – this is just an extrapolation of the existing rules. When creating your own maneuvers, you should avoid creating overpowered stunts. But, in some situations, it would be better to make overpowered stunts on purpose: for example, if a PC tries to trip a foe that has a wooden leg, he might get advantage. Sometimes, the upsides will be obvious: if you’re fighting over a bridge, disarming the enemy might mean he has no chance to get his weapon back.
Anything that makes the players pay attention to the surroundings and be creative instead of just looking to the character sheet is a good thing in my book. I would still limit these overpowered maneuvers to once per combat, lest it become repetitive and boring instead of creative and new.
Stunts for non-fighting classes
The problem of these stunts is that the only work for Fighter and similar classes. What about wizards and other classes that might have low Strength and low Dexterity? There are at least three possible answers to that.
First, it is okay for Fighters to have more combat maneuvers. This is what they are all about anyway. Let fighters have their time on the spotlight.
Second, you already have a universal combat stunt rule. You can easily adapt this to other abilities and skills: use Charisma (Intimidation) to make a foe frightened for a turn, or Dexterity (Sleight of Hand) to disarm him. Wizards can use Intelligence (Arcana) to blind a foe with an adequate cantrip, Wisdom (Animal Handling) to distract an animal, and so on.
This sounds enticing, but a bit unbalanced. It might make skill-based classes overpowered, specially when fighting big, dumb monsters. On the other hand, the Fact that Fighters get multiple attacks (and, thus, multiple maneuvers), might make up for the difference. Also, the stunt “costs” an attack, as usual, which means if you’re using a cantrip or special power to cause the effect, it must use a bonus action since the standard action is already taken. I’m undecided on this one; for now, I would probably consider most of those methods “overpowered maneuvers” like the ones described above, with similar limitations.
The third alternative is using damage as a gauge of effectiveness. One of the consequences of the “bounded accuracy” idea in Fifth Edition is that hit point became one of the most important ways of distinguishing low levels characters from high level ones, since proficiency and abilities change so little (or not at all, depending on the ability, class, etc.) over time. I reckon it makes sense that if an assassin has 100% chance of killing an ogre with a single hit, he should have an even better chance of disarming him.
Here is what you can do: the player states his intention and methods, makes an attack roll (or sneak attack, or spell, etc.), and rolls damage, but no HP is lost; instead, if the damage rolled is more than one-fifth of the target’s total HP, the stunt succeeds, leaving the target unarmed, blinded, etc., but otherwise unharmed.
Very abstract, I know – but it works very well, because of bounded accuracy. Using damage in this way might seem heterodox, but don’t worry, this is not really new – Steve Perrin wrote something quite similar in ’78…