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.
 * The Fighter Kensei Mastery III enhancement will add 1 to the threat range of a single specific weapon type, such as Kukri. This stacks with Improved Critical or Keen/Impact, but is applied second. With Improved Critical: Slashing and Kensei Kukri Mastery III, for example, an equipped Kukri will have a threat range of 14-20.
 * The Paladin Exalted Smite II enhancement will add 1 to the threat range of your weapon while active. Exalted Smite IV will add 2 while active. This stacks with Improved Critical / Keen, but is applied second.
 * The Sniper Shot ability granted by the Ranger Deepwood Sniper I enhancement makes a ranged attack with +1 to the threat range. This stacks with Improved Critical / Keen, but is applied second.

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