Deadly Strike

The Deadly Strike (DS) works exactly like the passive/magic skill Critical Strike (CS): it allows you to do double damage to any physical attack.

Though they work the same, DS and CS are mutually exclusive: they can't occur at the same time (re: quadruple damage). However, if a bowazon fails her CS check, she can then check for DS damage, and vice-versa. Furthermore, DS and CS aren't additive: you can't just add up your CS and DS and hope for 100%. According to most sources, if you want to find out your chances to inflict double damage per hit, the formula would look like this:

CS% + (DS/100 × (100 - CS)) = Your chance to do double damage.

Anything over 100% is, of course, redundant.

Thus, let's say a bowazon with slvl-5 Critical Strike (42% CS) has Gore Rider war boots (15% DS) and Guillaume's Face winged helm (15% DS); her chances for double physical damage would not be 72%, which is what you'd get if you added it up, but rather, would look like this:

42 + (30/100 × 58) ≈ 59% chance to do double damage.

Where can I get deadly strike?

Equipment:

Gore Rider war boots (15%)

Guillaume's Face winged helm (15%)

Highlord's Wrath amulet (24-37%)

Witchwild String short siege bow (39-99%)

Widowmaker ward bow (33%)

Horadric Blood gloves [magic helm/casque/armet + Ral rune + perfect ruby + any jewel] (5-10%)

Sockets:

Lo rune when socketed in a weapon (20%)

It is of some importance to note that all double damage is factored in before the physical damage is converted to another sort of damage; e.g., Magic Arrow, Fire Arrow, Cold Arrow.