Damage block

Damage Block is a passive ability that gives the wielding unit a chance to block some physical damage from an attack or ability.

Mechanics
Damage block has a chance to proc from any physical damage, including abilities that deal physical or composite damage, with the exception of Bristleback's Quill Spray and Leshrac's Diabolic Edict ability. Unlike armor, Damage Block will only reduce a set amount of damage, making it less useful over time. Damage Block will also occur before any armor reductions.

All items that use Damage Block mechanics use pseudo-random distribution, meaning the actual chance of blocking damage is only roughly 90% of the listed value, unless the listed chance is 100%.

Damage Block items will also block different amounts depending on whether the user is melee or ranged. The Poor Man's Shield will also have a reduced chance to proc from non-hero damage.

Stacking
Only one instance of Damage Block can proc at a time. The last acquired source of damage block is checked first, and only if it does not proc will any others roll. Items count as acquired when they are picked up and skills count when their first point is learned.

In other words, Damage Block follows a diminishing stacking pattern. For example, two Stout Shields would result in an 84% chance to block 20 damage on melee heroes. Three would result in a 94% chance, although it is a poor investment when compared to armor.

Sources of Damage Block

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