The Hallucinogenic Dart is acquired during the Sequence 4 mission 'Cable News'. It works the same as the rage darts in older Assassin's Creed games used to work - making enemies attack each other.
Take note that having four enemies under the influence is not enough - they have to be influenced with the same one dart (I tested it with separate darts and it does not work). There's a new tactic in this game that causes this: If you shoot a dart into a fire, the people standing around it will be affected. Technically you could try to go for this in the very mission where you get the darts but I could not get four enemies together near a fire. (If someone finds out a way this can be done or where the enemies gather naturally, please leave a note).
I found that a few memories down the road, there's a place where four enemies next to a fire happens naturally. In the "first" Sequence 5 memory, 'Friendly Competition', after you drive the omnibus you stop somewhere. In the place you need to be, there are a bunch of enemies, some standing still and some walking around. If you go to the roof of the building on the grounds (easily done without being noticed from where you start), you'll see a fire on the other side of the building with two enemies next to it. Two other enemies are walking around and will eventually stand next to the fireplace. Simply shoot a dart at the fire, then see the trophy unlock and watch as the carnage unfolds
