How do we deal with my DM's house-rule about critical damage?

Question

In the campaign I'm playing in, our DM has set a house-rule for critical damage. So, we do

maximum damage for one set of dice, roll the second set and add modifiers at the end

instead of the typical twice-the-dice and modifiers-at-the-end rule (PHB p. 196).

Example with a L1 Guiding Bolt spell:

• regular crit = 4d6 + 4d6, average of 28
• house-rule crit = 4×6 + 4d6, average of 38

It is great when you land a crit on creature and it's fun and exciting. But, when a creature lands a crit with a special attack or spell on one of us, it can easily wipe one of us out in one blow. Let alone area-of-effect damage! It is exciting and nerve-wracking. I really enjoy it, but I am the main healer. So, I'm regularly thinking: "Who will be the next one to pop their clogs?!" On one occasion I had an inkling that the boss we were facing was going to have a final deadly move before dying - it did. There was a massive explosion of energy and anyone close to it got killed. Luckily I had run into a corner of the room in my turn and was just out of range. The whole party was wiped but muggins.

What I'm looking for is a way of thwarting critical damage against our party, if it exists.

I am not looking for the obvious! We are already working on increasing our HP pools, using temporary HPs, reducing overall damage, increasing AC/saves, and having emergency supplies, spells and scrolls. We are also working together better, more tactically, doing things like spreading out and not putting ourselves in a line where possible.

Is there a way to prevent a creature from causing critical damage on a natural 20? Or, is there a class feature or feat that prevents critical damage specifically?

Answer 1

Adamantine armor

It has the property:

While you're wearing it, any critical hit against you becomes a normal hit. (DMG 150)

It is only classified as uncommon, but it is still a magic item. Depending on how the DM handles that, the difficulty of acquiring one may change. You also need to be proficient in at least medium armor to properly use one.

Note

Rolls with Disadvantage have a really low chance (1/400) to be crits. So if you impose it on an attack (eg. by dodging or the blur spell), you can improve your chances significantly.

Answer 2

Play as a Grave Cleric

The Grave Cleric domain from XGtE (p. 20) has the following ability at level 6:

As a reaction when you or a creature you can see within 30 feet of you suffers a critical hit, you can turn that hit into a normal hit.

This ability has a limited number of uses between long rests but unless you are getting critical hit too often, it can help mitigate the issue of critical hits doing some more damage than normal against your party.

Note, however, that in the particular situation of a critical hit against each party member at the same time, since this ability requires a reaction, the cleric would only be able to save a single character.¹

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^{1. Noted by Ryan C. Thompson}

Answer 3

Take the Lucky Feat

You can also spend one luck point when an attack roll is made against you. Roll a d20, and then choose whether the attack uses the attacker's roll or yours.

The Lucky feat allows, 3 times per long rest, a player to roll a d20 in addition to any d20s rolled as part of the attack. Afterwards, you get to choose whether your d20 or the target's gets used. In this context, it results in the attack effectively having disadvantage, so you will only take a critical hit if both your die and the attacker's are 20s. However, unlike other effects that typically apply disadvantage before the attack, you can choose to use this after the initial attack roll is made*. Multiple players can take the Lucky feat and each gain 3 uses of the ability, but only 1 use of Lucky is allowed per roll.

Additional benefits of this feat are that the Lucky rolls can also be used on your own attack rolls and saving throws, giving it more utility than simple crit-dodging.

Note that if the target is already attacking with advantage, Lucky does not let you pick amongst any 3 of the dice that were rolled. Instead, you are picking between your single die roll and whatever the attacker would have rolled anyways. (This is different from the other use of Lucky on your own attack roll where, if attacking with advantage or disadvantage, you have the choice of all 3 dice rolled).

_{*It is not entirely clear whether this effect of Lucky may apply "after [they] roll the die, but before the outcome is determined," but an existing question here suggests the answer is yes.}

Answer 4

I'd like to take a different approach to the other answers here:

Talk to the DM about removing the house rule for Monster critical hits

Monsters already have average damage values, rather than rolling. Average crits wouldn't be substantially different. The party impacts of a player critting are vastly different than a monster.

If a PC crits a monster for a bunch of damage, well the DM can do any number of things, from the monster dying and giving out XP to adding more HP or monsters or special effects. If the shoe is on the other foot, the PCs only options are to start dying and hope that it works out in the end. One of these is fun and the other can ruin a night.

Some examples

A CR1 bugbear normally deals 20 damage (not counting the surprise attack) on a crit, which is already a lot for even 3rd level characters. This houserule would bump it up to 27, enough to auto kill many 3rd level wizards.

CR3 Anklyosaurus normally deals 32 critical damage, and this would increase it to 42, more than enough to drop any member of a 4th level party (maybe not a Barbarian).

CR5 Bulette normally deals 56 critical damage, and this ups it to 78 critical damage. That would auto kill a 9th level wizard with 10 Constitution.

Theoretically (and in my experience), monster damage values are balanced around what the party can handle. Sometimes, the creatures in the Monster Manual (and others) bump up against the line, but there's a line. Sometimes, an increase doesn't hurt much, but sometimes it hurts a whole lot.