Deadly Strike

Deadly Strike is a chance to deal double physical damage with a hit, bare-handed included. Only the physical part doubles: the elemental damage on the weapon, Crushing Blow and Open Wounds are untouched. The per-level version grows with your character level (itemstatcost.txt op 2).

With Critical Strike

Critical Strike (the Amazon's passive, the Assassin's Claw Mastery, the Barbarian's weapon masteries) is the same double. The game checks the sources one after the other and the first success ends it, so they never stack into triple damage. With two sources the chance of a doubled hit is:

chance of a double hit = 1 − (1 − Deadly Strike) × (1 − Critical Strike)

Where it sits

Deadly Strike is step 7 of the attacker's order in Dealing Physical Damage: after the weapon's enhanced damage, the flat damage and the Strength, Dexterity and off-weapon multipliers, before the skill's own percentage. The Basin lists the skills it never applies to: Impale, Dragon Talon, Dragon Tail, Dragon Flight, Sacrifice and Smite — a Smiter's Deadly Strike is wasted. Physical damage converted to an element by Magic Arrow, Fire Arrow, Cold Arrow, Lightning Bolt, Fists of Fire or Berserk is still doubled.

A chance above 100% is wasted, and so is most of a high chance on a build that already has high Critical Strike: the second source only fills the gap the first leaves.

See also Crushing Blow, Open Wounds and Enhanced Damage.