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 roughly +10% fortification per level, with the most durable characters achieving +300% at level 30. 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
 * Warlock (with GOO pact), also Soul Eater

Feats

 * Epic past life - 10% per Fortification feat taken (Martial Sphere), up to 30%, when active
 * Iconic Past Life Feats (5% per Bladeforged life, up to 15%)
 * Improved Fortification (be aware of the side effect though)

Epic Destinies

 * Brace for Impact (20/40%)
 * Stand Against the Tide (5-25%)

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 (75%), 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
 * , set bonus on and

Penalty to fortification
Some effects reduce your fortification:
 * Yugoloth Essence of Cunning potion
 * Improved Sunder
 * Various monster only special attacks, such as Bebilith's Rend Armor

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%)
 * Animal Domain level 9 Cleric feat (15%)
 * Opportunist Rogue feat (10%)
 * Precision feat, toggled stance (25%)
 * Scion of Mechanus legendary feat (10%)
 * Grim Precision (5/10/15%)
 * Piercing Clarity (5/10%)
 * Trapsmith's Workshop guild buff (5%)
 * Advanced Sneak Attack (10%)
 * Mark of the Hunted passive(10%)
 * Silent Avenger set (5% heroic, 25% legendary, Artifact bonus to Armor-Piercing)
 * The Beast's Mantle three piece filigree set bonus (10%)

Sources of fortification reduction

 * Assassin's Trick (25%)
 * Cracking Attack (up to 20%}
 * Destruction (enchantment) (up to 15%), for example via
 * Insult (10%)
 * Improved Sunder feat (10%)
 * Shield of Condemnation (up to 50%)
 * Touch of Despair (dark, dark, dark monk finisher) (25%)
 * Mark of the Hunted active use(25%)

Race/type specific reduction

 * Deconstruct spell (25% for constructs)
 * Smiting Arrows (25% for constructs)
 * Weaken Construct items: -20% fort to constructs
 * Weaken Undead items: -20% fort to undead
 * Wrack Construct enhancement: up to -50% to constructs (Mechanic, Battle Engineer)

Bypassing immunity to sneak attack
In addition to fortification, monsters can possess "sneak attack immunity" that prevents sneak attacks (even at 100% fort bypass). Some special effects can remove that immunity:
 * Assassin's Trick
 * Wracking Shot
 * Shadowdancer capstone