User:HarveyPekar/Crit

A critical hit with a shortsword.

When a creature rolls a natural 20 on an attack roll, the attack is a critical hit. Critical hits automatically land regardless of the target's AC, and the attacker also rolls twice the normal number of dice to determine damage dealt.

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  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.

Maximizing 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.

On the other side of things, effects that add dice or trigger re-rolls of damage die can make the most out of 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.
•  Savage Attacker
•  Great Weapon Fighting
•  Organ Rearranger

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 is reduced by 2, available only in the The Dark Urge origin)

Consumables

• Elixir of Viciousness

Class features

•  Improved Critical Hit

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)

Guaranteed critical hits

There are also several ways to guarantee a critical hit.

• By applying  Paralysed (e.g. from any of  Hold Person,  Hold Monster) (attacks must be made from within  Melee: 1.5 m / 5  ft)
•  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)
• By applying  Sleeping (e.g. from any of  Sleep,  Glyph of Warding) (attacks must be made from within  Melee: 1.5 m / 5  ft)
• By applying  Exposing Bite (from  Wild Shape: Wolf) (once per Short Rest, for the next attack made from within  Melee: 1.5 m / 5  ft)