According to the manual 3.1 it does
Excruciating Strike improves the hero's attack ability. There's a 30% chance that an attack performed by the hero will deal double damage to a target.
but according to the information displayed in game and this site (http://www.heroesofmightandmagic.com/heroes5/demonlord_skills_abilities.shtml) it does
Excruciating Strike improves Mark of the Damned ability. There's a 40% chance that a Mark of the Damned performed by Demon Lord will deal double damage to a target.
That's outdated, 3.1 is the final version. Please use questions FAQ for future reference.
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