SRD:Coup de Grace (Weapon Enhancement)
From D&D Wiki
|This material is published under the OGL|
Coup de Grace: Coup de grace weapons are exceptionally dangerous. On a successful critical hit, the foe must succeed on a DC 27 Will save or be paralyzed for 1 round. While this ability does work on creatures that are immune to extra damage from critical hits, it does not work on creatures without an Intelligence score. Bows, crossbows, and slings bestow this ability on their ammunition.