You’ll find Devotee Outposts around Los Angeles after you beat the campaign of the Horizon Forbidden West: Burning Shores DLC. These are minor challenges Aloy can undertake when the real threat has been dealt with. It’s also a way to tie up loose ends storywise. Here’s where to find all three of them.
Devotee Outpost 1
You’ll find one Devotee Outpost east of Fleet’s End. In all three Devotee Outposts, you’ll find only a small number of enemies. However, one of them carries a Deathbringer Gun. Be sure to take care of that enemy first.
Devotee Outpost 2
There’s another Devotee Outpost on a small island north of Fleet’s End.
Devotee Outpost 3
The last Devotee Outpost is to the north, east of Heaven’s Rest.