- "It's another blade design... for delivering poison!"
- ―Leonardo da Vinci, upon deciphering a Codex page with the blade's design.[src]
Based on Altaïr's design in the Codex, it was later crafted by Leonardo da Vinci, for use by Ezio Auditore da Firenze. The weapon was essentially a thin, hollow needle hidden within the blade which could be filled with poison. Because of its exceedingly thin profile and the requirement of a hollow chamber, the Poison Blade risked being particularly brittle, and in his Codex, Altaïr cautioned that deviations from his exact schematics would likely produce a blade that could easily fracture.
Even more inconspicuous than the Hidden Blade, the Poison Blade could be used to inject a target with poison, causing them to attack anyone within range, before dying. Due to its delayed reaction, the Assassin was given much more time to escape than when using the Hidden Blade.
Besides dealing a direct kill to the intended assassination target, the Poison Blade could also be used as a distraction, as anyone within the immediate area would stop to watch the poisoned victim, creating an assassination opportunity if one of the spectators happened to be a target. Additionally, one could poison someone as a distraction to cover an approach to a restricted area in the same manner.
The Poison Blade was especially useful when completing assassination contracts, as in many cases, targets would either be followed by guards or within a guarded area. Poison vials could be bought from a doctor at 175 florins each, and up to five could be initially carried at a time. The maximum number of vials could be increased to ten or fifteen after buying poison vial upgrades, purchasable from any tailor.
This upgrade was a more potent poison that could be purchased from doctors after completing their shop quest. Fast Poison sent enemies immediately into a berserk state, rather than after a delay. As such, it was a more precise and quicker means of creating a diversion.
- Main article: Poison darts
Somewhat of a joint upgrade for both the Poison Blade and the Hidden Gun, poison darts allowed one to use the effects of the Poison Blade from a distance. The upgrade could be purchased from Leonardo, from one of his marked benches situated in various places around Rome.
- Silent kill: Guards would not notice the poisoning process, unless it was directly next to them and in their line of sight, though sometimes, even then, they would not notice.
- Attention: Distraction was an important tool to use during missions. If looking for a silent approach that smoke bombs could not provide, and could not find thieves, mercenaries, or courtesans within the immediate area, the Poison Blade could be used as an effective substitute. Poisoning anyone within ten meters of a guard's position would cause them, as well as all other guards near them, to move away from their position to where the poisoned victim was standing. Once the victim had drawn their attention, Ezio could either assassinate the guards and give away his position, or silently head on to the objective. It was important to leave the area before the poisoned victim had died, as the guards would start an investigation afterwards, and become suspicious of the Assassin if he was in the guards' line of sight.
- Clean kill: When performing an assassination contract, a single target could have at least one guard following close behind them. Ezio could poison the guard and move away into a hiding spot, while the aforementioned guard could inadvertently kill the target for him, eliminating the need to ever come out of hiding after the first administration.
- In Assassin's Creed II, the "Doctor" achievement could be earned by performing an air assassination on an already poisoned guard.
- If Ezio poisoned two guards who were close to each other, one might knock the other one out. In this case, the knocked out guard would not die (despite being previously poisoned), but would remain unconscious as if they were beaten in a fistfight.
- In the novelization of Assassin's Creed II, the Poison Blade could be used in combat for an instant kill. In the game, however, this was not the case, as the target died slowly and painfully.
- In Assassin's Creed: Brotherhood, the Poison Blade was not visible when unsheathing, aside from when Ezio was in a state of loading in the Animus, such as when equipping armor or dyeing robes, or within the Animus Virtual Training Program.
- Completing the "Venomous" shop quest would unlock Fast Poison, but in the process, it would also lock the standard poison in the "Supplies" inventory menu.
- When an archer was poisoned with regular poison and staggered over the edge of a building, they would not fall, but continue walking instead. However, once they started attacking, if they got close enough to the edge, they would then fall.
- In the novelization of Assassin's Creed: Revelations, the Poison Blade did not appear at all, though it appeared in the game.