At first glance the weapon appears to be a +1 short sword with a blade fashioned to appear as a venomous asp. When any of the following conditions are met, the blade’s unique qualities activate.
1. A successful attack with the Sneak Attack/Back Stab ability
2. The attacker rolls maximum damage.
3. A “critical” hit is rolled.
The blade of the weapon transforms into a venomous snake and burrows into the flesh of the victim. The snake does 1d6 damage every round and the victim must make a save versus Poison (each round) or suffer an additional 2d6 of damage. During this time the attacker will need to use another weapon since his blade is inside the victim. Once the victim dies or the snake is removed The Serpent Blade returns to normal.
Yeah, I just watched Conan.