Spell Damage

Spell damage is all damage which is not dealt by physical attacks. This includes all spells which deal magical, physical or pure damage, as well as all items with abilities that deal damage. Even the damage dealt by attack modifiers counts as spell damage, as long as the effect deals its damage in a separate instance from the attack 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.

Spell amplification
Spell damage can be directly amplified. Each point of increases a hero's spell damage by %, which equates to 1% spell damage for every  points of intelligence. Spell damage amplification of multiple sources stacks additively. The amplification does not only affect spell damage dealt against enemies, it also causes damage afflicted to allies or the own unit to get amplified.

These items passively increase the owner's spell damage amplification by merely having them equipped. They either grant a flat bonus, or bonus through intelligence.

Unaffected by amplification
There are certain cases of abilities that ignore spell amplification. It is important to note that those spells cannot spell lifesteal, too.

In general abilities related to health manipulation (instantly killing effect, health cost, delayed damage) and regular attacks (cleave, instant attacks) do not benefit from spell amplification. The exceptions to this rule are splash,, and which are still amplified.