Deadly Strike gives you an X% chance of dealing double Damage. The damage is doubled after all other bonuses, including Strength and Dexterity modifiers. Deadly Strike will not stack with the Assassin's, Barbarian's or Amazon's Critical Strike bonuses, although there are advantages to having both. When you have both Deadly Strike and Critical Strike the game will roll for one of the bonuses first(lets say Deadly Strike), if that fails the game will then roll for the other bonus(Critical Strike). If the first bonus succeeds, however, there is no roll for the second bonus as the game only allows for 2X damage, never 4X damage.