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.

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

Weapons

• Unseen Menace (while the weapon is  invisible)
• Bloodthirst → Improved Critical
• The Dead Shot → Improved Critical
• Knife of the Undermountain King → Organ Rearranger
• Duellist's Prerogative → Elegant Duellist (while off-hand is empty)

Cloaks

• Shade-Slayer Cloak → Stealthy Critical (while  Hiding)

Armour and headgear

• Covert Cowl → Covert Critical (while Obscured)
• Dark Justiciar Helmet → Covert Critical (while Obscured)
• Sarevok's Horned Helmet → Deathbringer's Sight

Conditions

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

Consumables

• Elixir of Viciousness

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.

Maximizing 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. 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:

Critical Hit Threshold	Chance without Advantage	Chance with Advantage	Two attacks with advantage
20	5%	9.75%	18.55%
19	10%	19%	34.39%
18	15%	27.75%	47.8%
17	20%	36%	59.04%
16	25%	43.75%	68.36%
15	30%	51%	75.99%
14	35%	57.75%	82.15%

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.
  •  Organ Rearranger rerolls damage die on critical hits, when rolling a 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 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 after killing a creature, 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

See also

• Advantage - For more on the calculations behind critical successes, misses, and advantage.