Damage types

All damage dealt in Dota 2 has a certain Damage type. Some are reduced by armor, some spell resistance, and some are seemingly not reduced at all. There are five damage types in Dota 2, and each interacts differently with modifiers and status effects such as armor, magic resistance, damage block, ethereal, magic immunity, and invulnerability.

Magical
Magical damage is caused primarily by spells, although not all do magic damage. Magic damage is reduced by Magic Resistance, does not affect magic immune units, and deals 1.4x damage against Ethereal units. The vast majority of abilities in Dota 2 deal magical damage.

All abilities not covered in the following tables deal magical damage.

Physical
Physical damage is caused by all regular attacks from all units, and only by certain skills. Physical damage is further divided into categories that modify damage based on the target. It is modified by both armor and damage block, is unaffected by magic resistance or magic immunity, and cannot affect ethereal units.

All physical damage is affected by both armor and armor type. Piercing damage, for example, deals 150% damage to unarmored units, such as lane creeps, but only 50% damage to heroes. These resistances are applied in addition to resistance from armor value. A full list of the attack types of creeps and summoned units can be found here.

The only exception to this is damage from Cleave, which is reduced by armor type but not by armor value or damage block. Practically speaking this means armor is worthless against cleave damage. Splash (from ranged attacks) is reduced as normal.

Abilities
Certain spells deal physical damage instead of magical. Physical damage from abilities are treated as Hero damage, and deal full damage to all units except for structures and siege creeps. Damage from abilities can be reduced by armor value and damage block, will not trigger abilities that check for attacks such as Counter Helix, and cannot be evaded. The following abilities deal physical damage:
 * Damage cannot be reduced by damage block.

Pure
Pure damage is a form of magical damage that is unreduced by magic resistance or magic amplification (it always does exactly the damage it says it does). Because it is magical, it also ignores all armor or forms of damage block. It does not affect units under magic immunity and does normal damage against ethereal units. The following abilities deal pure damage:
 * Whirling Death will deal magical damage if there are no trees within a radius of 300.

Composite
Composite damage (also known as mixed damage) is a physical damage that is also reduced by magic resistance. It is reduced by armor, magic resistance, and damage block, but goes through magic immunity. It does not affect ethereal units. The following abilities deal composite damage:
 * If the Spirit Bear dies, Lone Druid takes backlash damage from the unit that killed the bear.

HP Removal
HP Removal works by directly subtracting hitpoints from the affected unit. It ignores all damage reductions and affects units under magic immunity and does normal damage against ethereal units. HP removal will also damage through invulnerability, with the exception of Invoker's EMP. Unlike all other damage types, HP removal is not seen as damage by the game, so it will not interrupt Healing Salve or trigger any other on-damage effects such as Spiked Carapace. The following abilities count as HP removal damage:
 * Soul Rip does HP Removal to all units counted towards its damage/heal value.

The following abilities cause HP removal damage to the caster:

Universal
Universal damage is very similar to magical damage. It is reduced by magic resistance and deals 1.4x damage to ethereal units, but unlike magical damage, it goes through magic immunity. Currently there are only three spells with this damage type: