Threat range

Each weapon has a threat range for critical hits.

A critical threat is obtained when you roll a d20 to hit your opponent and the result (before applying any bonus or penalty) is within the threat range of your weapon (even if your modified attack roll wouldn't beat the opponent's AC).

A critical threat means you have a chance to score a critical hit. To do so, a confirmation roll is made, with the same to-hit bonus being applied (plus extra confirmation bonuses such as Seeker, Kensei enhancements, Fighter Critical Accuracy, etc.); in this roll, 1's do not automatically fail to hit. This roll must beat the target's AC in order to "confirm" the critical hit.

In DDO, a confirmation roll can still succeed on a roll of 1; it does not auto-miss on such a roll.

If that roll also results in a hit against the target's AC, the critical hit is confirmed, and the base damage is rolled once for every critical hit multiplier. If the confirmation roll is a miss, then the attack is not a critical hit and is rolled as a normal attack (IE, auto-success on natural 20 or test vs. AC for non-20).

All weapons create a critical threat on a roll of 20, but some have a larger threat range, extending down to 19-20 or even 18-20.

Ways to increase threat range

 * The Keen (slashing/piercing) or Impact (bludgeoning) weapon prefixes double the weapon's base threat range. A Kukri (base threat range 18-20) with Keen, for example, will have a threat range of 15-20. This does not stack with Improved Critical.
 * The Improved Critical feat doubles the base threat range for any equipped weapon in a specific category (slashing/piercing/bludgeoning/ranged/thrown). This does not stack with Keen/Impact.
 * Competence bonus to threat range can come from various sources. It affects weapon's base multiplier and thus the effect is doubled by Improved Critical feat or Keen enchantment. Multiple effects granting competence bonus don't stack.
 * Fighter's Keen Edge enhancement adds 1 to the threat range of a his/her selected weapon group.
 * Bard's Swashbuckling enhancement modifies the threat / multiplier as follows
 * Dagger, Shortsword, Throwing Dagger: +1 Threat, +1 Multiplier
 * Light Mace, Light Hammer, Kama, Sickle, Dart, Shuriken, Throwing Axe, Throwing Hammer: +2 Threat, +1 Multiplier
 * Kukri, Rapier: +1 Multiplier
 * Handaxe: +2 Threat
 * Light Pick: +1 Threat
 * Staff Specialization, available for Rogues and Monks, add +1 to thread and multiplier for quarterstaves
 * Monk's Ninja Master enhancement modifies the threat as follows
 * Shortsword: +1 threat
 * Kama, Shuriken: +2 threat
 * Legendary Dreadnought's Pulverizer adds +1 threat for bludgeoning weapons. Find details and exceptions in the LD article.
 * Sacred bonus (?) to threat range is available to Divine Crusaders: +1 threat for all weapons (not handwraps/animal forms). The effect is doubled by Improved Critical feat or Keen enchantment.
 * Insight bonus to threat range is available from Bard's Exploit Weakness enhancement. Every time you don't crit, you get a +1. The effect is doubled by Improved Critical feat or Keen enchantment.
 * Barbarian's Critical Rage gives a stacking +2 threat while raging. This bonus is NOT doubled with Improved Critical/Keen. (Inconsistent, might be different for thrown weapons )

Many active attacks, often coming from enhancements, feats, or epic destiny abilities, modify the critical threat and/or multiplier of a single attack. These effects usually stack with everything else. For example:
 * The Paladin's exalted smite
 * Legendary Dreadnought's Cleave-like attacks
 * Fury of the Wild's Adrenaline

Weapon damage comparison
Note that this calculation does not include additional damage from strength modifier, weapon enhancement bonus, weapon prefix/suffix effects, etc, resulting in x2 multiplier weapons looking better than they really are.

i.e:
 * a strength 10 (+0) fighter with vanilla weapons, no improved crit
 * Bastard Sword: 5.5 base expected, 11 on crit &rarr; 6.1 avg
 * Khopesh: 4.5 base expected, 13.5 on crit &rarr; 5.4 avg
 * a strength 30 (+10) fighter with +5 weapons, no improved crit
 * Bastard Sword: 20.5 base expected, 41 on crit &rarr; 22.55 avg
 * Khopesh: 19.5 base expected, 58.5 on crit &rarr; 23.4 avg