Magic. Sorcery. Playable on a sorcery-using character facing a non-automatic-attack. Cancels the attack or gives the attack -3 prowess, your choice. Unless he is a Ringwraith, character makes a corruption check modified by -4. Cannot be duplicated against a given attack.
"...as if...smitten with a shower of bolts and huge slingstones..."
-LotRVI