Critical hit

A critical hit with a shortsword.

Critical hits occur when a creature rolls a natural 20 on an attack roll. Critical hits automatically land regardless of the target's armour class, and the attacker also rolls twice the normal number of dice to determine damage dealt.

While initially a mechanic subject purely to the whims of luck, a number of features, abilities, and items allow the player to manipulate the odds of scoring a critical hit. Combined with effects that make critical hits themselves stronger, building around critical hits can be a valid strategic option.

Mechanics

Any ability that uses an attack roll, including spell attacks, is capable of landing critical hits. Any dice that are rolled for damage, including additional dice such as those from smites or combat manoeuvres, are rolled twice. Abilities with multiple attack rolls like Slashing Flourish or Scorching Ray can have multiple critical hits, with each attack rolling independently of one another.

Flat modifiers and bonuses to damage – including the creature's relevant ability score modifier and proficiency bonus – are not doubled. Creatures whose attacks deal a flat amount of damage rather than a die roll, such as Familiars, can roll critical hits but do not deal extra damage on them.

Reducing critical threshold

Some feats, passive features, and items reduce the critical hit threshold by 1, allowing the creature to land critical hits by rolling either 19 or 20 on attack rolls. Multiple sources of this effect stack, allowing the critical hit threshold to go even lower than 19.

Equipment

Conditions

A Most Bloody Inheritance - Critical threshold reduced by 2, available only in the The Dark Urge origin.

Consumables

Elixir of Viciousness - Reduces the number needed to roll a Critical Hit by 1.

Class features

Improved Critical Hit - Level 3 passive for the Champion Fighter subclass.

Feats

Spell Sniper - Spell attacks only.

Illithid Powers

Fierce Perilous Stakes - Target's critical threshold is reduced by 5, but it is also vulnerable to all damage.

Maximising critical hit value

Increasing likelihood of critical hits

Any effect that triggers a re-roll of an attack die will increase the likelihood of landing a critical hit. Attacking with Advantage almost doubles the likelihood of rolling a critical.^[2] Features that allow for situational rerolls, like Halfling Luck, similarly increase the odds. Reducing the critical threshold increases chances of scoring a critical hit accordingly; taking into account advantage and multiple attacks, the odds of hitting at least one critical with a character in one round of combat are as follows:^[3]

Critical Hit Threshold	Chance without Advantage	Chance with Advantage	Two attacks with advantage	Three attacks (two main+off-hand) with advantage
20	5%	9.75%	18.55%	26.49%
19	10%	19%	34.39%	46.86%
18	15%	27.75%	47.8%	62.29%
17	20%	36%	59.04%	73.79%
16	25%	43.75%	68.36%	82.20%
15	30%	51%	75.99%	88.24%
14	35%	57.75%	82.15%	92.46%

Additional effects on critical hit

On the other side of things, some effects add add additional value to critical hits when they occur. These include:

Brutal Critical and Savage Attacks add an extra damage die to the damage of the attack, i.e. an attack dealing 1d10Bludgeoning will critically deal 2d10 + 1d10Bludgeoning. Additional damage dice, like those from battle manoeuvres, are not subject to this additive bonus, with the exception of Divine Smite.
Effects that reroll damage rolls will see greater benefit on critical hits, since the raw number of damage rolls increases:
- Savage Attacker rerolls all damage die twice.
- Great Weapon Fighting rerolls damage die for two-handed melee weapons, when rolling a 1 or 2.
Effects that provide a unique bonus when a critical hit occurs:
- Paralysing Critical paralyzes a target on critical hit, once per long rest.
- Mortal Reminder possibly frightens enemies on critical hit.
- Great Weapon Master: Bonus Attack enables an attack using a bonus action, after a critical hit.
- Spellthief restores a level 1 spell slot on critical hit.
- Dolor Amarus, Vicious Battleaxe, and Vicious Shortbow add a flat damage boost of 7 to critical hits.
- Sword of Life Stealing adds a flat damage boost to critical hits and adds temporary hit points.
- Intransigent Warhammer knocks creatures prone in a radius on critical hit.
- Craterflesh Gloves adds 1d6Force to critical hits, itself doubled as it is adding an additional damage die.
- Blightbringer inflicts Slowed on critical hit.
- Clown Hammer inflicts Hideous Laughter on both target and user on critical hit.

Guaranteed critical hits

There are also several ways to guarantee a critical hit.

Melee-range attacks against Paralysed or Sleeping targets
- Includes Hold Person, Hold Monster, and Clench of the North Wind
Luck of the Far Realms once per Long Rest
Killer's Sweetheart once per Long Rest after killing a creature
Assassinate:Ambush against Surprised creatures
Scarlet Saturation once per Long Rest after consuming the +3 bonus from Scarlet Remittance
After applying Exposing Bite, on the next attack made from within melee range
Next attack made with a weapon enchanted by Wild Magic: Enchant Weapons

Negating critical hits

Some equipment prevent attackers from landing critical hits on the wearer when worn. When a natural 20 is rolled on an attack roll against the wearer, the roll is simply treated as a 20 while still being affected by the wearer's armour class. With a high enough armour class while wearing equipment that prevents critical hits from being landed, it is possible to always be missed by attack rolls, even if a natural 20 is rolled.