My players want to use called-shots on Strahd

Question

Possible spoilers for the D&D 5E adventure Curse of Strahd:

So last week my players encountered Strahd at level 3. My intention with this is to teach the players to think before spending their valuable resources. Strahd would not have killed them right away - he was only toying with them and testing their strength. But then one of my players said "I have wooden stakes, I want to stake him in the heart."

We are all new players and I am a new DM. Since 5e has no rules for called shots, I told them that it would be such a difficult hit that they needed to roll a flat 20 on the dice for a critical hit. They then started to "fish" for a critical hit with two weapon fighting and 2 wooden stakes each. I felt like it was a poor way of handling this situation

They've also tried to "call shots" before. The war cleric wanted to hit an enemy on the legs to make it fall prone and the way I handled it was telling them that the battlemaster archetype of the fighter has a similar feature called maneuvers and it would be a boring class if everyone was be able to do it.

So my question here is: how do you handle it when your players want to call shots?

Answer 1

For this particular scenario, it's simply not possible.

From the Vampire (added bold for emphasis):

Stake to the Heart. If a piercing weapon made of wood is driven into the vampire's heart while the vampire is incapacitated in its resting place, the vampire is paralyzed until the stake is removed.

Notably (and players should be informed of this if they do not know), a stake to the heart does nothing special to a Vampire if they're just out and about. And that's aside from the basic impossibility of trying to drive a hand-powered wooden piercing weapon all the way into someone's heart in the middle of combat.

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For other scenarios (like hitting an enemy in the legs to knock prone), there are often rules that govern those actions.
The Shove action can be used to knock an enemy prone, but is a skill challenge and does not do any damage.
The Disarm action is listed in the DMG as a variant rule, and also is a skill challenge and does not do any damage.

Many other actions can be resolved with some sort of skill challenge, but also don't be afraid to just say no to "called shots" where the player will expect either an instakill or an incredibly debilitating effect.

Answer 2

First Priority: Make sure the Rules don't offer a Solution

I want to call attention to the example of the Cleric trying to knock an enemy prone, because you're framing it as the Cleric's player "trying to call a shot", but the 5th Edition rules offer a direct mechanic for knocking creatures prone: the Shove Action.

Using the Attack action, you can make a special melee attack to shove a creature, either to knock it prone or push it away from you. If you're able to make multiple attacks with the Attack action, this attack replaces one of them.

The target of your shove must be no more than one size larger than you, and it must be within your reach. You make a Strength (Athletics) check contested by the target's Strength (Athletics) or Dexterity (Acrobatics) check (the target chooses the ability to use). You succeed automatically if the target is incapacitated. If you succeed, you either knock the target prone or push it 5 feet away from you.

—Shoving a Creature, Player's Handbook, pg. 195

Just to clear up confusion, this kind of attack is substantially less good than the feature given to Battlemaster Fighters, which is better because

• It deals [a lot of] damage; a Shove deals no damage at all
• The [Strength] Saving Throw the target needs to make is harder to achieve than the ability contest made with a Shove, which allows the target to use either Strength or Dexterity, maximizing the odds they have a stat that will counter the shove.

If a player tells you they want to do something unorthodox, first check the books, since a lot of "unusual" player actions in combat have perfectly cromulent rules in place to handle them, regardless of whether the player is familiar with the rules enough to know what those rules are.

Strahd specifically

Strahd cannot be directly killed by a stake to the heart. Vampires in 5th Edition borrow many of the tropes of classical vampires, but for a Vampire in 5th Edition to be killed, it's not sufficient to stake them unless you've already successfully reduced them to 0 hit points, and stake them while they're rejuvenating in their resting place. Players are welcome to try to deal as much damage as possible with whatever tools they think will get the job done, but no amount of heart-stabbing, with or without stakes, is sufficient against a Vampire that hasn't been reduced to 0 hit points yet.

If you genuinely [think that you] need rules for how to handle "Called Shots" in general, I recommend viewing other posts on the topic, like this one, to better understand the hazards of those kinds of house-rules. But when it comes to important enemies like Strahd or other powerful boss-tier creatures/NPCs, you need to first make sure you understand the rules for how these characters can be defeated in the first place. You'll avoid these kinds of Calvinball-y situations if you have a better mastery over the scenario that's being presented to the players.