Fortification

Fortification represents a percentage chance to negate additional damage incurred from a critical hit or sneak attack.
 * When a critical hit is negated, damage is applied normally rather than at the critical hit multiplier. Weapon enchantments that deal additional damage on a critical hit apply fully even if a crit is negated (for example Holy Burst or Bloodletter).
 * When a sneak attack is negated, the extra damage is not applied.

In DDO, most players strive for 100% fortification around level 10. For end game epic elites, 150%+ is sometimes recommended. Most monsters don't have fortification, unless it is granted by the race/type (Undead, Constructs...). Quest and raid bosses have higher fortification, but usually not full.

Players and monsters can gain abilities to bypass a portion of fortification.

Sources of Fortification
Fortification items stack with racial fortification of Warforged. Standard fortification items provide enhancement bonuses to fortification and do not stack with each other. Likewise, two exceptional fortification items don't stack, as they provide insight bonuses to fortification and insight bonuses do not stack. One regular fortification (enhancement bonus) and one exceptional fortification (insight bonus) item will stack with each other because they are different bonus types.

Races and racial enhancements
 * Bladeforged
 * Warforged

Class abilities and enhancements
 * Sacred Defender
 * Pale Master undead forms
 * Elemental forms of Druids and Sorcerers...)

Epic Destinies
 * Brace for Impact from Unyielding Sentinel
 * Stand Against the Tide stance from Unyielding Sentinel

Items
 * Enhancement bonus - most common source
 * Named items
 * Random items with the Fortified prefix (20% - 125%)
 * Legacy items (before Update 19) with Light (25%), Moderate (50%), Heavy (100%) fortification, used to be a suffix on random armors, belts, shields, and rings
 * Augment: Sapphire of Light/Moderate/Heavy Fortification
 * Insight bonus - stacks with enhancement bonus
 * Construct Fortification items
 * Exceptional Fortification items
 * Other stacking bonuses
 * Fabricator's Ingenuity, set bonus on and

Bypassing Fortification
Since Update 13, monsters with sneak attack dice are able to bypass some of the player fortification equal to monsters' Challenge Rating. E.g.:
 * If a CR 1 Kobold Sneak attacks someone with 50% fortification, the Kobold Sneak has a 51% chance to normally conduct a critical hit or sneak attack.
 * If a CR 20 Lava Mephit attacks someone with 100% fortification, the Lava Mephit has a 20% chance to normally conduct a critical hit or sneak attack.
 * If a CR 54 Sellsword Footpad attacks someone with 150% fortification, the Sellsword Footpad has a 4% chance to normally conduct a critical hit or sneak attack.
 * If a CR 29 Earth Elemental attacks someone with 125% fortification, it is still going to have a 0% chance to critically hit or sneak attack, as it has no sneak attack dice.

Players can bypass a portion of enemy fortification (personal benefit), or generally reduce enemy fortification (universal benefit).

Sources of personal fortification bypass:

 * Items with Armor-Piercing enchantment (10/15/20%/35%)
 * Opportunist Rogue feat (10%)
 * Precision feat, toggled stance (25%)
 * Grim Precision, Shadowdancer T3 ability (5/10/15%)
 * Piercing Clarity, Grandmaster of Flowers T4 ability (5/10%)
 * Trapsmith's Workshop guild buff (5%)

Sources of fortification reduction:

 * Destruction (enchantment) (up to 15%)
 * Insult, Fury of the Wild T3 ability (10%)
 * Improved Sunder feat (10%)
 * Shield of Condemnation, second core ability from Angel of Vengeance enhancements (up to 50%)
 * Touch of Despair (dark, dark, dark monk finisher) (25%)

Race/type specific reduction:

 * Weaken Undead items: -20% fort to undead
 * Weaken Construct items: -20% fort to constructs
 * Wrack Construct enhancement: up to -50% to constructs (Mechanic enhancements T3, Battle Engineer enhancements T2)

Discussion on stacking: