The darts had the same effect as the Poison Blade, but were fired from a distance in the same way as the Hidden Gun. They were, however, completely silent, which made them extremely useful during stealth missions. Aiming and firing a dart was an almost flawless motion, similar to that of the crossbow.
The effects of a poison dart occurred immediately after the target was shot:
First stage: The target stood at their post or walked alongside their group of guards, though they would soon begin to clutch at their chest and struggle to remain standing as the poison's effects set in. None of their fellow guards seemed to notice that something was wrong with the target at first.
Second stage: The poison began to have serious effects on the target. They began to flail helplessly in a circle, striking anyone who came within their reach. If they had a weapon, they may have drawn it and flailed it around as well. Civilians and guards alike were drawn in, making it a perfect distraction, though they kept enough of a distance to not be hit unless surprised.
Third stage: The target was overcome completely by the poison and fell on the ground, still experiencing the effects caused by the toxin running through their veins. After a short time, the target died, after the poison had sapped away all of their energy.
From a long enough distance, Ezio would not be noticed by guards even if he shot their comrades. However, they would attack him if he was close enough and they noticed him firing the dart. Poisoning guards, particularly Seekers with their long weapons and moderate health, provided a good distraction that could hold any nearby guards' attention long enough to bypass them. Following this, firing a dart at a target from amongst a crowd of civilians would not draw attention, creating an opportunity for a kill whilst blending.