Does spell power affect max-health-percent based damage?

Some abilities, such as Malthael's Reaper's Mark, deal damage based on the maximum health of their target. Is this damage ability or "spell" damage, and if so, is it affected by spell power modifying abilities?

Solution 1:

While traits (such as Reaper's Mark) are counted as Spell Damage, Malthael's trait damage is health % based, so it isn't affected by any modifiers (not even armor). There is, essentially, three types of damage in HotS: Physical, Spell and Percent.

Percent is not affected by any modifiers (Armor, Spell or Physical). See it on HeroesWiki

Percent damage, or Percentage-based damage, is a special category of Spell damage that resembles other MOBAs' 'Pure' damage type. All in-game Abilities and Talents that deal damage based on a percentage the target's maximum or current Health do Percent damage. Percent-based damage is dealt in Spell damage, but is flagged to ignore the Armor system and damage modifiers entirely. This means that percentage-based Abilities and Talents will deal the raw amount of damage regardless of the amount of damage Buffs or Debuffs on the attacker or the Armor state of the target. Percent damage is capped to 36.5 damage per percent of Health for an instance of damage, Scaling 4% per caster's level. The cap is set to limit Percent-based damage's effectiveness against high-HP Battleground Objectives such as the Immortals on Battlefield of Eternity. The limit is high enough for it to not affect any Heroes of roughly the same level in a normal game situation.

The claim that Armor doesn't reduce it either can be found in the Armor Page

Damage that is based on a percent of a given Hero's Health cannot be mitigated by any form of Armor. This means that Giant Killer (+1.5% Max health damage per attack) will do full damage to the target regardless of their current Armor state.

See also how other traits are carefully worded to not include any % damage:

Lunara's Basic Attacks and damaging Abilities poison their target, dealing 36 damage a second for 3 seconds. Every additional application increases the duration by 3 seconds, up to a maximum of 9 seconds.

Nazeebo's Basic Attacks and Abilities poison Non-Heroic enemies, causing them to take 66 additional damage over 6 seconds.

So while both Lunara's and Nazeebo's traits will scale with Spell Power (and be reduced by Spell Armor), Malthael's won't. Any other % traits, like Giant Killer, won't either.