In Assassin's Creed II, Leonardo da Vinci designs the poison blade upgrade for Ezio's hidden blade.
The poison blade lets Ezio dose a target from close range without immediately starting a conventional fight. After exposure, the victim becomes disoriented and eventually dies, allowing the Assassin to create an opening or eliminate a target discreetly. The upgrade is presented as one of Leonardo's inventions based on his study of the hidden blade and its mechanisms.
The weapon belongs to the broader family of hidden-blade improvements introduced in Ezio's trilogy. Assassin's Creed II also gives Ezio a ranged pistol integrated into the blade mechanism, but that is a separate upgrade with a different function. The poison blade should likewise not be confused with poison darts or later equipment from other games. Leonardo's role is fictionalized within the series, though the historical Leonardo really was an inventor and artist active in Renaissance Italy.