To 'complete' a territory, you need to finish all mysteries and collect all wealth and artifacts.
There are 4 main territories in Svartalfheim. There are at least 15 viewpoints to help you discover most/all the wealth, mysteries and artifacts. Some artifacts are glitched and may not appear on the map by activating the nearest viewpoint. You may have to physically wander around until the icon appears.
(if you need help with specific things, the links are to guides provided by PowerPyx.com)
GULLNAMAR (Level 340) - 26 wealth
, 7 mysteries
, 15 artifacts
SVALADAL (Level 355) - 31 wealth
, 11 mysteries
, 21 artifacts
VANGRINN (Level 370) - 30 wealth
, 12 mysteries
, 14 artifacts
EITRI (Level 390) - 6 wealth
, 1 mystery
, 5 artifacts
Complete all of those to get the achievement.
Wealth includes ingots, Armor and Weapons, Books of Knowledge, Silica raid rewards, etc.
Mysteries include World Events, Dwarven Altars, Mythical Memories, Dwarves in Distress, etc.
Artifacts include Jotun Blights, Heroic Sagas, Uncut Gemstones, Cosmetic Schemes, etc.
During your completion of this achievement, you should also get
If you discover and enter all the shelters while scouring the map, you'll get