This achievement is linked to the mission "In a Strange Land
" that you should receive after killing six members of The Order of Ancients, Vicelin, Avgos, Sister Frideswid, Hunta, Leofgifu and Kjotve (the rightmost horizontal branch). Several of which are linked to the completion of the Lunden Arc, and unlocking the following achievement
Once you have the mission, you'll be tasked with killing another member of The Order of Ancients by travelling to a new location. Once you set sail, you will arrive in Vinland, and unlock the achievement immediately after the first conversation.
It's worth noting this achievement didn't unlock for me when it should have. Luckily, I knew this achievement should have unlocked and reloaded an auto save just before leaving England. I suggest you make a save before leaving to travel to Vinland, just in case the achievement doesn't unlock the first time around for you.