Well, my guess is visit all the new location or complete all 3 task at the same route.
Spoilers for the achievement below:
I did a little bit of digging into the code to see what the locked achievement is and it has the internal name 'all_hinterlands'. As far as I can tell, it is supposed to be unlocked when you have visited all of the locations in the hinterlands across all runs, but it fails to unlock because it checks if you have visited as many locations as there are total locations, but there are two unused (?) locations in the game files, the "Strange shack" and the "Truck stop" (which was mentioned to have been scrapped in another thread).
So it doesn't seem like a lore achievement, I think all of the main paths in the new content have been found and discussed here!