The Chant

This can be a little miss-able during a normal play through, but if you explore just a little bit more you should be able to get this. Most areas you explore in the game will gave Cultists roaming around or ones you have to battle anyways. These are enemies outside the "Gloom" areas and have masks on. Use your fire weapons to deal extra damage and take these out. A little after the lighthouse I got this by taking out every Cultist I saw. You can always pause the game and check your stats to see where you are at with this.

My best advice is whenever possible explore the area a bit more, many ares you back track through in the game will have 1-2 Cultists hanging out for some kills. Some will also only spawn when you open a door or move to a certain area in the level/