Needs review
Source: Sandstorm

Description: Having observed the ways of a desert viper, you have learned to use ki in a fashion similar to poison.

Prerequisites: Improved Unarmed Strike, Stunning Fist, Improved Initiative, WIS 15

You must declare that you are using this feat before you make your attack roll (thus, a failed attack roll ruins the attempt). By expending one of your Stunning Fist uses for the day, you imbue your next unarmed attack with disruptive ki that mimics the effects of mild viper venom. If that attack hits, it deals normal damage and forces the target to make a Fortitude saving throw (DC 10 + 1/2 your character level + your Wis modifier). A creature that fails the saving throw takes 1d3 points of Constitution damage and must make a similar saving throw 1 minute later or take another 1d3 points of Constitution damage. This feat can be used only once per round.

Special

A fighter may select Rattlesnake Strike as one of his fighter bonus feats.