The Viper Steel Sword is a Steel Weapon that is also a piece of Witcher Gear. This particular weapon's diagram can be obtained from the Side Quest: Scavenger Hunt: Viper School Gear, along with the diagram for the Viper Silver Sword.

The diagram for this weapon is found in a Bandit Camp in some ruins near the Ransacked Village (look for a fallen pillar to climb the cliff up to the ruins). The chest in the main part of the ruins holds the diagram, as well as a note with a clue to the other diagram.