I'm guessing the Deflect Missile edge. Help file inside.

DEFLECT MISSILES (2585)

Assassins with this edge have their senses, reflexes, and agility honed to so
great a degree they can snatch or deflect missiles meant to harm them from
the air. This includes hurl, all archery-based attacks, iceneedles, most orc
snares, fire seeds, reanimated limbs, grenades, and so on. They have a small
chance to do this normally, but the success rate increases drastically while
they sustain the perfect calm of a martial trance.