To 'complete' a territory, you need to finish all mysteries and collect all wealth and artifacts. When I played, some things seemed to be restricted until the story let me get to them. But
LuckyKant says they were able to get all collectibles before advancing the story at all.
There are 4 main territories in this part of Francia. All are Level 200. There are 9 viewpoints.
(if you need help with specific things, the links are to guides provided by PowerPyx.com)
Melunois - 8
wealth, 7
mysteries, 2
artifacts Paris - 9
wealth, 3
mysteries, 3
artifacts Amienois - 12
wealth, 5
mysteries, 2
artifactsEvresin - 11
wealth, 5
mysteries, 2
artifacts Complete all of those to get the achievement.
Mysteries include altars, cairns, events, side quests, Noble fights, etc.
Artifacts include hoard maps, flying tattoos, cursed symbols, etc.
Wealth includes Tungsten ingots, Armor/Gear, Books of Knowledge, raid chests, etc.