List of sources of critical hit threshold reduction

Normally only an attack roll of 20 is a critical hit. The following is a list of ways to make critical hits more likely.

Increasing the likelihood of critical hits

• Attacking with Advantage almost doubles the likelihood of rolling a critical
• Similarly, any effect that triggers a re-roll of an attack die will increase the likelihood of landing a critical hit, including:
  •  Halfling Luck

Reducing the threshold for critical hits

These effects reduce the threshold required for a critical hit by 1.

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

Consumables

• Elixir of Viciousness

Class features

•  Improved Critical Hit

Feats

•  Spell Sniper (attacks with spells only)

Illithid Powers

•  Perilous Stakes (Reduced by 5 but 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