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Defense vs. missiles

From Diablo 2 Wiki

This special modifier is found on several uniques in the Expansion. It's usually written, "+ defense vs. missiles", such as the "+100 Defense vs. Missiles" on the Cat's Eye unique amulet.

The function is fairly simple; these items simply add that much defense, as though it was coming from an item of armor or a charm, but that value is only checked against missile attacks. (The added amount may be far more than 100, since it is modified by dexterity, enhanced damage mods, skills, etc).

This property helps to lower the to/hit of attacks such as arrows, quills, spears, and projectile elemental attacks from Abyss Knights. Does not work on damage over time spells like Firewall or Poison clouds.