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, with the exception of Tidebringer which is only triggered if the ability is off cooldown. 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. If an attacking unit misses, whether by evasion or blind, units in the area will not take damage. Damage dealt by cleave is reduced by armor type and damage block, but not armor value and can't be evaded. It is derived from the actual damage of the attacking unit (as opposed to the damage that the primary target took from the attack). This means that in most cases, Cleave damage is not reduced at all, but it still won't work on ethereal units. Cleave does not affect the primary target of the attack. 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.

It is possible for a hero to have more than 100% cleave, such as by having three or more Battle Furies. This results in units within the cleave's area of effect taking more damage than the initial target of the attack.

Cleave with Unique Attack Modifiers and Buffs
Cleave does not extend the attack effects (including unique attack modifier) or buffs that the attacking unit has. Cleave is also considered one of attack effects, thus doesn't stack with others. For example, if a Hero with Cleave has Desolator, only the primary target of the attack will receive the effect. The only buff placer that affects Cleave is Critical Strike. If you proc a Critical Strike when you cleave, the damage multiplier will affect all units hit by the 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.