Spell Damage

Spell damage ( $$Sp$$ ) is usually damage dealt in a separate instance from attack damage, with some exceptions. This includes all abilities (including item abilities and attack modifiers) which deal the 3 damage types — magical, physical or pure damage.

Magic spell damage is influenced by magic resistance, physical spell damage is influenced by armor and pure damage is not affected by either. All three underlie damage manipulation.

Stacking
All sources of spell damage manipulation stack multiplicatively, and are categorized into two categories — Outgoing Spell Damage Manipulation and Incoming Spell Damage Manipulation. Since these are handled separately, the same category stack additively while different categories stack multiplicatively with each other respectively.

For example, spell damage dealt by summons, unless stated, it is not affected by the outgoing spell damage amplification of the hero. However, it can be amplified by incoming spell damage amplifications.

Spell Damage Multiplier is defined as:



With  being the Spell Damage Multiplier,   the spell damage amplification sources and   the spell damage reduction sources.


 * Example 1

Outgoing Spell Damage Manipulation
A unit that has outgoing spell damage manipulation deals more or less spell damage with any spell damage they apply to any unit. Multiple sources of outgoing spell damage manipulation stack additively.



Item Sources
Many items grant their owners a spell damage amplification bonus by giving them a direct bonus. The effects are limited to the item's owner, which must have the item equipped. 

Incoming Spell Damage Amplification
A unit that has incoming spell damage amplification debuff on them takes more spell damage, and multiple sources of incoming spell damage amplification stack additively. Incoming spell damage amplification affects every source of spell damage, except for damage flagged as HP Removal.

Exceptions
There are certain cases of abilities that ignore both types of spell damage amplification, and those abilities usually cannot spell lifesteal. In general, abilities with health manipulation mechanics (Instant Kill, HP Removal, Delayed Damage etc.) are not affect by both types of spell damage amplifications.

However, the following abilities are not affected by outgoing spell damage amplification, but they are affected by incoming spell damage amplification:

Patch History
Dano mágico Урон заклинаний 技能伤害