As you progress through the third chapter you'll hear of a group who call themselves the Cult of Kosmos. Tracking down and killing these cult members becomes a primary objective.
In order to take on cult members, you’ll need to gather clues left by individual cultists to find their identity. Once found, you'll be able to fully reveal who they are and ultimately their location.
The Eyes of Kosmos cultists are probably the quickest members (Top left branch / 6 members)
Names and Locations (Provided by Powerpyx):
Note: Thanks to APOLLO2342 for confirming Sokrates missions don’t show up until after chapter 4.
Elpenor: Unmissable, story related.
Sotera: She’s inside the Fort of Megaris, usually walking around the docks or on the right side of the Fort. You can also use your eagle to scan for her.
The Master: Unlocked during “A Life’s Worth” Side Quest (Attika Region, must do all Sokrates side quests in the region until it pops up). Then he’s standing around in the south of Attika Region, between the Fast Travel Point “Tower of Sounion” and the Silver Mountain. You can again use your eagle to find him more easily.
Hermippos: Free Speech Side Quests (Attika Region, must do all side quests in the region until it pops up). You kill him during that quest.
Midas: In Argolis Region. You should have him unlocked as soon as you get the Cultist List and can track him from there.
Nyx the Shadow: She’s standing around in the south-east of Attika city. You can again use your eagle to look for her.