Cleave

Cleave causes a melee hero's attacks to deal a portion of their damage in a radius in front of them.

Mechanics
Cleave is triggered every time a unit with Cleave attacks. Cleave does not affect the primary target of the attack. Damage done is determined by the actual damage of the attacking unit, not the damage that the primary target takes from the attack. Damage dealt by cleave is reduced by armor type and damage block, but not armor value. Cleave is determined by a circular area, with a radius of the Cleave's range, and a center located the range of the Cleave away from the attacker in the direction of the primary target.

If a Hero has multiple different sources of Cleave, they will work independently. Each source will deal separate damage to the affected units, but will not deal extra damage to the target of the attack.

Cleave with Unique Attack Modifiers and Buffs
Cleave does not extend the effects of most unique attack modifier or buffs that the attacking unit has. For example, if a Hero with Cleave has Desolator, only the primary target of the attack will receive the effect. However, Critical Strike and the 150 bonus damage from Shadow Blade will affect cleave. Because Critical Strike alters the actual damage of the attacker, if it is triggered by the initial attack, the damage dealt to all affected units will be multiplied.