From DDO wiki
Using this melee special attack, you may stun the target for 6 seconds. Stunned creatures are considered helpless. A successful Fortitude save negates this effect. (DC: 10 + Strength modifier + other modifiers)
- Stunning Blow doesn't work on red named, purple named or creatures of the wrong genus.
- You may increase the DC of your stunning blow attacks with various enhancements and by using weapons with the stunning suffix.
- A Fighter may select this feat as one of his fighter bonus feats.