Leonardo da Vinci gives Ezio the Poison Blade mechanism in Assassin's Creed II.
Leonardo is a recurring ally who studies and improves Assassin equipment. Among the devices he provides is a poisoned version of the hidden blade, allowing Ezio to apply poison to a target from close range or at a distance, depending on the game's upgrade system.
The Poison Blade is distinct from the Hidden Gun, another Leonardo-related improvement that lets Ezio fire a projectile from his bracer. Assassin's Creed II also introduces other tools and weapons as Ezio's abilities expand across Renaissance Italy.
The hidden blade itself is the Brotherhood's signature weapon, but the question concerns the specialized poison mechanism. Leonardo's inventions are fictionalized within the game, even though Leonardo da Vinci was a real Renaissance polymath.