Death blow is a special ability of Dread Knights, which gives them 20% chance of dealing double base damage when attacking a creature stack. The game information states that Dread Knights would deal double damage when the death blow occurs, but that is not technically correct as it only doubles the base damage. In other words, bonuses from Offense, Luck or attack-defense difference are not doubled. See damage calculation for more specific information about the matter.
Creatures with death blow ability: