This requires you to collect all the Roman Artifacts in the game and give them to the museum in your home village. There are 52 in total. You can complete any time after reaching England and building the Roman art museum, I waited until after completing the main story.
Here is the list of all 52: https://www.powerpyx.com/assassins-creed-valhalla-all-roman-...
If you prefer to find them naturally over the course of your playthrough, these artifacts are mostly found near ruins in cities and also in the countryside. Set your "Exploration Difficulty" to "Adventurer", white dots representing artifacts will show up when you're 500m away. But the white dots will disappear from your map if you get too far away. To keep it on your map permanently, you need to be within 50m and use Odin's Sight
to scan it and have it be recognized.
If you see a question mark (?) icon on your map, you can pay that person 30 silver to locate a nearby artifact.
The achievement will unlock as soon as you finish giving them all to the museum in your home village.