Cleave

Cleave causes a melee hero's attacks to deal a portion of their damage in a radius in front of them. Ranged units cannot cleave.

Mechanics
If a unit has cleave, its attacks deal damage in a circular area in front of the target upon attack. Only melee attacks of units can cleave, ranged attacks can never cleave. An attack is classified as melee when the attack does not rely on projectiles. The primary attack target is not hit by the cleave damage. An attack has to actually hit the target in order to cleave. A missed attack, be it caused by evasion or a blind effect, does not cleave. Cleave hits invisible, but not hidden or invulnerable units, and also does not work against wards and buildings.

Damage

The damage is entirely based on the attacking unit's attack damage. It does not matter how much damage the attacked unit actually took as cleave only checks the damage value of the attacking unit. The damage is not reduced by the armor value, only the armor type and damage block reduce it. Cleave damage is always physical, so it never affects units in ethereal state, and always pierces spell immunity. It is also increased/decreased by damage manipulation.

Radius

A cleave's effect radius is shaped like a circle. This circle is infront of the attacking unit, so that the attacking unit is always at the edge of the circle. This means that cleaves reach always twice as long as their tooltips say, since the value in the tooltips always tells the radius of the circle.

Stacking

Cleave is an attack modifier which fully stacks with any other modifier. It does not interfere with any attack modifier. This means that for example the armor reducing effect of Desolator is still only applied to the primary attack target. However, if an attack modifier increases or reduces damage, the modifier will affect the cleave damage, even when the modification is not visible in the damage value in the hud.

Attack damage modifying effects which are not shown in the hud include all critical strikes and. These do affect the damage of cleaves.

The damage from is the only added attack damage which can be cleaved. All other sources of added attack damage (like with all bashes) are not cleaved.

Cleave Stacking

Cleave also fully stacks with other sources of cleave. Each source of cleave works fully independent. Their damage is not summed up, each of them deal their own instance of damage.

As an example, with level 4, buffed with a level 4  and 2 Battle Furies will deal 4 instances of damage on each attack to units within 200 range infront of him, 3 damage instances to units within 200-280 range and 1 instance to units within 280-300 range. This is because in this scenario, one source has a radius of 200, 2 of them have 280 radius and one has 300 radius, so all 4 hit units within 200 range, but only one of them hits units up to 300 range away. In this scenario, units within 200 radius take 186% damage from cleave, units between 200-280 range take 136% damage and units between 280-300 take 66% damage.

Splash
Splash is in some ways the ranged version of cleave. It works in a similar way as cleave does, with the difference being that the damage is dealt in a radius around the attacked unit, not in a circle in front of the attacking unit. When attacking with splash, the primary target of an attack does not receive the splash effect. If an attack misses, no splash damage is applied in the area (except for, in which case 50% of the damage is applied in the area). Splash works with attack modifiers the same way as cleave does.

Unlike cleave, splash damage can deal Physical or Magical damage. As such, splash effects are reduced by the armor value, the armor type, damage block and ethereal state if Physical, and by magic resistance as well as spell immunity if Magical. They are also affected by damage manipulation.

Many splash effects have multiple areas of effect, each dealing a different percentage of the attacking unit's attack damage, meaning that units closer to the primary target take more damage than units further away.