Links take you to text walkthroughs on gamepur.com. They tell/show how and where to find each tomb, and tell how to complete them. Video is from Gamers Heroes on YouTube.
To start this DLC, go to the Ravensthorpe docks and look west. Talk to the people watching the animals frolicking in the river. You'll then need to use your feathered friend to scout across the river and reveal a glowing area where you'll find a rune to interact with. This opens a cave entrance to the first tomb.
Manius' Sanctum You should get this achievement for finishing this first area, which is quite easy. I finished it
before the achievements went live, so I had to finish one of the other tombs to get this afterward.
If you did not get the achievement after Manius' Sanctum, then read further.There are 3 main tombs after the Sanctum (there will be more when future updates add them). You can do these in any order. Finish ANY 1 to get this achievement.
Boudicca Tomb (East Anglia)
Cassivellaunus Tomb (Sciropescire)
Venutius Tomb (Eurvicscire)
If you finish any 3 of the above Tombs (Sanctum+Tombs), you will get this (videos for these 3 tombs in this other solution)
Don't forget the
hidden artifacts also. You can collect these 3, but more will be needed to open the glowing door in Manius' Sanctum, after more tombs are added to the game in the future.