Viper venomous silver sword is a silver sword and is part of the Viper School Gear introduced in the Hearts of Stone expansion for The Witcher 3: Wild Hunt.
However, unlike other items in the Viper School Gear set, this one does NOT have a diagram that can be received without using console commands (or, theoretically, mods), and the sword can only be obtained during the quest Whatsoever a Man Soweth... by challenging Master Mirror. The sword is found protruding from a stone in Gaunter's world.