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In our last session (with me as the DM), a creature was surrounded by the party and was trying to flee. She cast Levitate on herself to try to float up and get away. As she was going up and thereby out of reach of several party members, they each got an Opportunity Attack against her. Several of them wanted to use this opportunity attack to try to grapple, holding on to her to keep her from getting away, rather than a more traditional melee attack. I wasn't sure whether this was an allowed way to take an Opportunity Attack, but the scene of her floating up with one of the player characters grabbing onto her legs as she went up was pretty neat, and I figured via Rule of Cool that I should just allow it. And it did lead to a really fun encounter, with the character dangling off the legs of this creature in midair trying to slap some manacles on her to prevent further spellcasting.

Now, after the session, I figured I'd look up the actual rules, and sure enough by the book Opportunity Attacks only allow for a melee attack, not a grapple. So I guess my question is, what issues might I encounter if I maintain this precedent of allowing a more generous set of actions as Opportunity Attacks? It sure seems that trying to tackle or trip somebody as they run past you could make for some fun scenes, and it feels more "realistic" in some sense than only allowing for a melee attack.

I'm a relatively novice DM, and am thus hesitant to go very far outside of the standard rules.

If I just allow for this -- substitute in a Grapple for a melee attack during an Opportunity Attack -- will this create balance problems?

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This should not cause a balance problem if you confine it to Grapple.

Grappling

When you want to grab a creature or wrestle with it, you can use the Attack action to make a special melee attack, a grapple. If you’re able to make multiple attacks with the Attack action, this attack replaces one of them. (Basic Rules p. 74)

A couple of points on using a grapple attempt versus a weapon attack:

  1. The creature takes no further damage (and is thus still alive to do stuff next turn/round)
  2. The Grapple attempt may fail, just as any Opportunity Attack may fail.
  3. The character trying to Grapple Must Have A Free Hand to do so! (A sword and shield, or two weapon wielder, won't have a free hand for their reaction, but they can drop one of the held items).

Action economy wise, the player had a reaction to spend and an attack available. Making "a special melee attack" versus "making a melee attack" is a subtle distinction. It is still "making a melee attack" that does not damage, and cannot critically hit.

If you are playing in a game where characters are not using feats, it's a way for martial characters to get a little more flexibility in the action economy.

Recommendation

Keep doing this for the next few levels, restricting it to the grapple action as "a special melee attack." If it gets out of whack it should become evident during play. Make sure your players are informed that this will be "play test" to see how this fits. Make sure enemies try this as the occasion arises. ( And see how your players react to that).

Leave Shove out of this for the time being

Why? The enemy is already moving out of the way. If you are doing battle near ledges you can see how a shove off of the ledge might be a more powerful effect (falling damage can ramp up rather quickly). However, if you are happy with how Grapple works out, consider adding this "special melee attack" as a reaction ... but beware of the can of worms.

Shoving can lead to moving a creature, or knocking them Prone. Prone, as a condition, opens up attacks with advantage for your team, etc, and applies some other effects. While it too can fail, and does no damage, its effects are more pronounced than Grapple's.


Rule of Cool

As an aside, love the rule of cool / rule of fun approach you took in-play to make a ruling that was fun, and then looked into details later.

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This steps on the toes of the Sentinel Feat

The problem with allowing a grapple as an Opportunity Attack is that it allows a character to completely prevent movement away. The Opportunity Attack as it stands now makes moving away risky, but once you allow it to prevent the movement away at all, you are introducing a mechanic that is typically only found with the Sentinel Feat (PHB, Chapter 6, Feats).

The Sentinel Feat gives a character the ability to (among other things including damage and an additional reaction source):

When you hit a creature with an opportunity attack, the creature’s speed becomes 0 for the rest of the turn.

Should there be a cost?

A grapple replicates that bullet point but without the cost of picking that feat when picking an ASI/Feat. Given the "cost" of that, allowing it at no cost is definitely overpowering.

This creates a new attack option that didn't previously exist. If everyone has access to it, a strict cost isn't necessary because it's a standard action, but please see my final section on what the 'cost' may end up being for the PCs.

However, an alternate is to create a cost for being able to do this. This way it's not necessarily something that is done all the time, but something that someone can do because they invested in the ability to do it. In order to assess that, we can look at the mechanics of the Sentinel Feat.

Do note that Feats are optional, so even having this potential ability was an option for the designers (it isn't possible to do anything like this without the Feat.) If you would like to implement at your table, it does need a cost balance to emulate that of the feat cost (as it seems the cost to do this was high enough for the designers to incorporate it into an ASI.) You could probably do something somewhat less as you don't get the rest of the feat, but it does need some sort of balance if you do this.

In addition, because of the standard OA rules, character who don't wield a weapon and don't have warcaster don't get an OA. This gives them one where the designers felt (because they didn't provide) that those character types shouldn't get an Opportunity Attack and now they would.

Melee Attack vs Grapple

Another piece to consider is that the standard OA gives a potential to hurt a creature attempting to withdraw with a single attack. This potential also comes with a 5% chance to be a guaranteed miss.

A grapple avoids that entirely AND it also provides a means for you to use your full Attack when it's your round(and other friendlies) because the creature never left. That is HUGE. Normally, you'd have gotten a single attack and then the creature is out of melee range. Allowing this (and assuming successful), leaves the creature in range where many more attacks will occur the next round.

Consider what happens to players in this case as well

The players may want to do this to their enemies, but it's also going to happen to them as well if it becomes legal at your table. THat means any time they want to run, they may not be able to - and that will most likely result in a player unconscious or player death. The risk is most likely higher for the players than the enemies for this occurring during battle.

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As the other answers have already pointed out, there are potential balance problems with allowing grapples to occur on an attack of opportunity. However, I feel like an obvious consideration/alternative has been overlooked:

Attacks, including Opportunity Attacks, will trigger Concentration Checks

Levitate is a second-level spell that requires concentration. If even a single attack from the PCs lands (which is likely if all of them get Opportunity Attacks at the same time), the creature will need to make a concentration check on their levitation spell, and if it fails, they fall back down to the ground. And since the levitate spell has a max height of 20 feet, and their horizontal speed is limited by their ability to push/pull themselves along walls/ceilings (RAW), it's very easy for the land-bound PCs to keep firing ranged attacks at them, until a hit brings them down.

I wouldn't regret the call you made: it sounds like you and the Players had fun, which is ultimately the goal of any game. But in the future, you can tell the players that they have options for bringing them down, and that the creature levitating away doesn't mean they just get to escape for free.

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