Damage Manipulation

There are several ways to affect the final value of a damage instance. Damage can be manipulated on both ends, from the dealing unit (outgoing damage) and the receiving unit (incoming damage). Armor and magic resistance are the most common damage manipulators, affecting incoming physical and magical damage respectively (no similar trait exists for pure damage). They usually reduce damage, but can also amplify it if their values are negative. Spell damage amplification is the main trait for amplifying (but not for reducing) outgoing spell damage (no similar trait exists for attack damage). Besides these, there are some other ways to manipulate damage, ranging from amplifying damage, reducing damage and even negating damage completely.

Amplifying incoming damage
Damage amplification causes afflicted units to take additional damage.

Besides these, other ways to amplify damage is by reducing armor for physical damage or magic resistance for magical damage.

Amplifying outgoing damage
Besides spell damage amplification, Bloodrage is the only ability able to amplify outgoing damage.

Reducing incoming damage
Just like how damage can be amplified, it can also be reduced.

Besides these, other ways to reduce damage is by increasing armor against physical damage or magic resistance against magical damage.

Reducing outgoing damage
Silver Edge's Shadow Walk is the only ability able to reduce outgoing damage.

Stacking damage reduction & amplification
Armor and magic resistance work separately from other damage manipulators. This means they always stack multiplicatively with any other listed ability.

Besides 's aura, 's rum and, all damage reducing and damage amplifying effects stack additively with each other. Borrowed Time, Ghostship and Mana Shield stack multiplicatively with them. Mana Shield also stacks multiplicatively with Borrowed Time and Ghostship. However, the damage reductions of Borrowed Time and Ghostship do not stack with each other, although both still react on the damage (Borrowed Time still heals Abaddon, and Ghostship's rum wear-off still counts the damage). The damage amplification which illusions usually have stacks multiplicatively with all other damage amplifying and reducing effects (considering they are able to affect illusions).

Examples of additive stacking (all values consider highest possible value, examples ignore armor and magic resistance, 100% is the base value):

100 + 40 + 50 - 20 = 170% damage
 * A hero affected by, and  takes:

100 + 30 - 22 - 50 = 58% damage
 * Spectre affected by, and   takes:

100 - 80 - 20 = 0% damage
 * Ursa affected by and  takes:

100 - 100 + 40 + 30 + 50 + 30 = 150% damage
 * A hero affected by, , , and  takes:

Examples of multiplicative stacking:

(100 - 20 + 50) * 40% = 52% damage
 * Medusa affected by, and  takes:

(100 - 20 + 25) * 40% * 50% = 21% damage
 * Medusa affected by the same as above, but also additionally by takes:

(100 - 20 - 50) * 400% = 120% damage
 * A illusion (taking 400% damage) affected by  and   takes:

and are the only abilities able to affect outgoing damage. They stack additively with each other. Since incoming and outgoing damage are separate, they stack multiplicatively with each other.

Damage negation
Damage negation causes entire instances of damage to deal no damage to its target, however, the instance is still registered. This means, if something reacts on damage without having a minimum damage threshold, it still reacts on the negated damage.

Preventing on-damage effects
Many abilities react on damage a unit receives. In some cases, fully negating the damage causes them to not proc. However some on-damage effects also react on 0 damage so they still may proc even when the proccing damage got negated.

The following abilities and effects do not react on damage when it gets fully negated (reduced to 0):

1 Includes all sources of lifesteal, except, and 's Spell Lifesteal, which are listed separately.

2 These work when the damage gets negated by.

3 These abilities have damage counters, which do not work when the damage gets negated.

4 's and 's "pre-heal" mechanic does not work when the damage gets properly negated, otherwise it would cause units to get healed.

5 Despite not reacting on damage which was already blocked by another source of damage negation, it still loses a charge per damage instance when the other source of damage negation was.

The following abilities still do react on fully negated damage:

1 Both cleave and splash damage from react on fully negated damage.

2 The damage of and the damage spread of  are fully unaffected by any form of reduction, except for magic resistance and armor.

Pre-Heal
Pre-Heal, just like regular damage negation, prevents a unit from dying. However, the way in which that is achieved is completely different. Instead of directly reducing or negating damage, this mechanic rather manipulates the health of the unit.

A pre-heal only activates when the unit is going to receive a damage instance which exceeds its current health. If that happens, the unit's current health gets increased equal to the amount of the incoming damage plus one, right before the damage hits. This causes the unit's health to land exactly at 1. This also works when an incoming damage instance is greater than the unit's maximum health. In this case, its maximum health gets increased as well and immediately reverted to default after the damage hits.

This means that technically, the incoming damage is not reduced at all. This is the reason why the abilities which use this mechanic do not prevent any on-damage effects from triggering, unlike regular damage negation.

The following abilities use this pre-heal mechanic: