Dusk Diver 2

Story-related and cannot be missed.

You'll get this automatically as part of the main quest, early on during Chapter 2, "Overture". The "Possessed" are normal people who live in Ximending who somehow got possessed by a Chaos Beast. They're quite obvious, as you'll notice that they have a lot of phantom-like black shadows surrounding them. When you get too close to one, you'll immediately be pulled into a small Youshanding pocket (basically similar to games that have specific battle screens like old Final Fantasies, rather than out in the world) where you'll fight a small handful of Chaos Beasts.

NOTE: Although this is the only trophy directly for killing Possessed citizens, you'll have to kill 100 Possessed for the Collection Quest, "Abbot's Chaos Beast Dispatch", which you'll automatically get in Chapter 4 upon unlocking the Ritual locator. (Thankfully, any Possessed you killed prior to receiving the side quest will still count towards it.)

Although the Possessed stop showing up on your map later on, once you unlock the "Ritual" feature in Chapter 4, you'll be able to Pray at the Temple once per Chapter to perform a "Ritual", which will mark the location of certain special Possesseds on your map. These are higher leveled Possessed that give more and better rewards.