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
Competence bonus to threat range can come from various sources. Bonuses to critical threat range usually are not doubled Improved Critical. Multiple effects granting competence bonus don't stack.
 * Assassin enhancements T5 - Knife Specialization: +1 threat for daggers, doubled with Improved Critical: Piercing feat. +1 multiplier for daggers and kukris.
 * Battle Engineer enhancements T5 - Reconstructed Armaments multiselector: +1 to chosen weapon type.
 * Deepwood Stalker enhancements core 4: +1 for all weapons (not shields/unarmed.)
 * Henshin Mystic enhancements T5 - Staff Specialization: +1 threat, +1 multiplier for quarterstaves.
 * Horizon Walker enhancements T5 - Feywild Attunement: +1 threat, +1 multiplier for longbows and shortbows.
 * Inquisitive enhancements core 4: +1 threat, +1 multiplier for light and heavy non-repeating crossbows.
 * Kensei enhancements T5 - Keen Edge: +1 for selected focus weapon group.
 * Ninja Spy enhancements T5 - Deadly Striker: while centered, +1 with any weapon (except handwraps and quarterstaves,) +2 for kamas and shuriken.
 * Occult Slayer enhancements T5 - Occult Metalline: +1 threat, +1 multiplier
 * Ravager enhancements T5 - Critical Rage: +1/+2
 * Shintao enhancements T5 - Empty Hand Mastery: +1 threat, +1 multiplier for handwraps.
 * Sorcerer Eldritch Knight enhancements T5 - Force's Edge: +1 for 12 seconds after casting an offensive spell.
 * Swashbuckler enhancements core 2 - while wielding a finesseable or thrown weapon in your main hand, a buckler or nothing in your off hand, and wearing light or no armor, modifies the critical profile 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 Unlike most bonuses, Swashbuckling bonus to threat is doubled by Improved Critical.
 * Thief-Acrobat enhancements T5 - Staff Specialization: +1 threat, +1 multiplier for quarterstaves.
 * Vile Chemist enhancements core 5: +1 threat, +1 multiplier for simple weapons while wielding a simple weapon, orb, rune arm, or nothing in your off hand.
 * Vistani Knife Fighter enhancements core 4: +1 threat, +1 multiplier for daggers and throwing daggers.
 * Warpriest enhancements core 5: +1 for favored weapons.
 * War Soul enhancements core 5: +1 for favored weapons.
 * Wizard Eldritch Knight enhancements T5 - Force's Edge: +1 for 12 seconds after casting an offensive spell.
 * Deep Gnome enhancements T4 - Gnomish Weapon Training: +1 for light hammers, light maces, light picks, shortswords, shortbows, light crossbows, and light repeating crossbows.
 * Gnome enhancements T4 - Gnomish Weapon Training: +1 for light hammers, light maces, light picks, shortswords, shortbows, light crossbows, and light repeating crossbows.
 * Halfling enhancements T4 - Master Thrower: +1 for thrown weapons.
 * Paladin T4 spell - Holy Sword: +1 threat, +1 multiplier for all weapons (not shields.) Also available as Cleric War Domain power - SLA.

Insight bonus

 * Swashbuckler enhancements T5 - Exploit Weakness: stacking +1 every time you damage an enemy but do not critically hit.

Morale bonus

 * Knight's Training feat: +1 for Battle Axes, Heavy Maces, Morningstars, War Hammers. +2 for Battle Axes if you have Improved Critical: Slashing.
 * Swords to Plowshares feat: +1 for Kamas, Sickles, and Handwraps. +2 for Quarterstaves.

Untyped / unclear bonus type

 * Shintao enhancements T5 - Violence Begets Violence: while centered, stacking +1 for 6 seconds when an opponent misses you in melee. Stacks up to 5 times and is reset if you critically hit.
 * Vanguard enhancements core 5: +1 threat, +1 multiplier of your shield.
 * Blessing of Silvanus: +2 for Mauls.

Many active attacks, often coming from enhancements, feats, or epic destiny abilities, modify the critical threat and/or multiplier of a single attack or short duration. These effects usually stack with everything else. For example:
 * Knight of the Chalice enhancements T3 - Exalted Smite: +0/1/2 threat, +1/1/2 multiplier.
 * Sacred Fist enhancements T3 - Exalted Smite: +0/1/2 threat, +1/1/2 multiplier.
 * Vistani Knife Fighter enhancements T5 - Vendetta: +1 threat, +1 multiplier to daggers or throwing knives for 15 seconds.
 * Fury of the Wild T1 epic strike multiselector - Adrenaline: +4/8/16 to your next melee attack. Requires a melee weapon.
 * Legendary Dreadnought's Cleave-like attacks
 * Shiradi Champion T1 epic strike multiselector - Hunt's End: +16 to your next shot with a ranged or thrown weapon.

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. This calculation also assumes a 100% critical confirmation roll, in this case resulting in lower threat weapons (which usually carry a better multiplier) looking better. To understand the breaking point, besides the obvious critical effects, consider how often you hit an opponent. The harder it is to hit (attack roll vs AC), the better it is to have a higher threat range weapon as you always hit on a non-confirmed critical.

STR modifier example: (on hit with vanilla weapons, does not consider glancing blows, feats or enhancements)
 * a strength 10 (+0) fighter
 * Bastard Sword: 5.5 base expected, 11 on crit → 6.1 avg
 * Khopesh: 4.5 base expected, 13.5 on crit → 5.4 avg
 * a strength 30 (+10) fighter
 * Bastard Sword: 15.5 base expected, 31 on crit → 17.05 avg
 * Khopesh: 14.5 base expected, 43.5 on crit → 17.4 avg
 * a strength 40 (+15) fighter
 * Bastard Sword: 20.5 base expected, 41 on crit → 22.55 avg
 * Khopesh: 19.5 base expected, 58.5 on crit → 23.4 avg