Which Assassin's Creed game introduced the hidden blade's poison dart launcher?

Assassin's Creed II introduced the poison dart launcher as an upgrade for the hidden blade.

In Assassin's Creed II, Leonardo da Vinci helps Ezio Auditore improve his equipment. One of the additions is a poison blade mechanism that lets Ezio poison targets from a distance. This expands the original hidden blade's role from close-range assassination into a more flexible stealth tool.

The poison dart launcher is distinct from the pistol introduced in the same game. The pistol is also a Leonardo-assisted ranged weapon, but it creates more noise and is used differently. Remembering that distinction prevents a common mix-up when listing Ezio's equipment.

Later Assassin's Creed games develop ranged stealth weapons in different ways, including darts, blowpipes, and specialized launchers. However, the poison blade upgrade belongs to the Renaissance-era equipment set introduced by Assassin's Creed II, rather than the first game starring Altaïr.