If I succeed in grappling a spellcaster, can I choose to cover their mouth so they can't cast a verbal spell? If so, do they have to break the grapple to pull my hand away from their mouth?
This isn't covered by the grappled condition
The grappled condition states:
- A grappled creature’s speed becomes 0, and it can’t benefit from any bonus to its speed.
- The condition ends if the grappler is incapacitated (see the condition).
- The condition also ends if an effect removes the grappled creature from the reach of the grappler or grappling effect, such as when a creature is hurled away by the thunderwave spell.
Nothing here refers to the ability to extend the grapple to removing a wizard's speech. This activity would fall under a DM ruling.
Here's how I would rule it
You make a special melee attack that uses your Strength modifier (perhaps you could gain a bonus with the Tavern Brawler feat). On a successful Strength contest (where the opponent can choose to use Strength or Dexterity) the creature cannot use verbal components until it makes a Strength check contested by a DC equal to your Strength score as an action. You would of course need to be able to reach the mouth and have a free hand (if you want to silence that giant warmage start climbing).
Do note that some spells do not have a verbal component and sorcerers have subtle spell to work around this tactic
A grappled creature can speak and perform verbal components
However, a grapple is only one example of a Contest in Combat
Battle often involves pitting your prowess against that of your foe. Such a challenge is represented by a contest. This section includes the most common Contests that require an action in combat: grappling and shoving a creature. The GM can use these Contests as models for improvising others.
If the GM thinks that holding a creature's mouth shut is possible, then they could improvise a contest to do just that.
Here is an example of a jaw hold contest that is modeled after a grapple check and which I think would work well:
When you want to prevent a creature from speaking, you can use the Attack action to make a Special melee attack, a jaw hold. If you’re able to make multiple attacks with the Attack action, this attack replaces one of them.
The target of your jaw hold must be already grappled by you. Using another free hand, you try to hold the target's mouth shut by making a jaw hold check instead of an attack roll: a Strength (Athletics) check contested by the target’s Strength (Athletics) or Dexterity (Acrobatics) check (the target chooses the ability to use). If you succeed, the target cannot speak until the grapple ends or until you release the jaw hold (no action required).
Preventing verbal spells is a very strong effect. Which is why my example requires you to sacrifice at least two attacks (one for the grapple and one for the jaw hold) and both hands (one for the grapple and one for the jaw hold).
Grapple, no. The grapple condition only holds the target in place. A "grapple" in 5e is not a wrestling move to restrain a target. ALL a "grapple" is, in 5e, is grabbing the target with a hand or appendage so it can't run away. That's it. ALL there is. Finito. It isn't a choke, or a way to prevent any activity other than movement.
If you can do something that applies the "restrained" condition, I would allow that to be aimed at preventing speech.
Grappled & restrained—these conditions interfere with a spell only if it has a somatic component and the caster's hands are bound. #DnD
This is covered under the Ability Checks section of the PHB, and complemented by Reactions.
PHB pg. 173, emphasis mine:
An ability check tests a character's or monster's innate talent and training in an effort to overcome a challenge. The DM calls for an ability check when a character or monster attempts an action (other than an attack) that has a chance of failure. When the outcome is uncertain, the dice determine the results.
For every ability check, the DM decides which of the six abilities is relevant to the task at hand and the difficulty of the task, represented by a Difficulty Class. The more difficult a task, the higher its DC. The Typical Difficulty Classes table shows the most common DCs.
And then there's this:
PHB pg. 190 under Reactions, emphasis mine:
Certain special abilities, spells, and situations allow you to take a special action called a reaction. A reaction is an instant response to a trigger of some kind, which can occur on your turn or on someone else's.
As DM it's certainly your prerogative to rule that the situation is appropriate to allow for a reaction, followed by a contest to attempt to stifle the verbal component of the spellcasting.
Now, I would caution against allowing just any grappled creature from being able to be stifled. It's very situational, as I've highlighted here. Just because I grabbed onto a sorcerer's cloak as I closed with him/her doesn't mean that I'm in a position to stop them from talking. Whereas if I walk up behind somebody at a bar who's trying to start a fight with my friends, a grapple can easily be described as a chokehold, putting me in a situation where I could easily stifle any verbal components.
This will encourage descriptive roleplaying, and I think it will encourage your players to be very creative in their contests to see if they can leverage things towards opening up opportunities. I've had a lot of success with encouraging descriptive combat in my games, but always ensuring that players are aware of the limitations of each of their abilities. For instance, a player can't do a grapple and subdue verbal components with the same action. If the player wants to initiate a contest after the grapple, they'll have to use one of their other attacks or an action/reaction later to do so. No preemptive reactions off of no triggers.