Northgard

This is the unique relic of the Boar clan so start a new game, pick the Boars and play as normal. They have a pretty good bonus that each sector you own increase your population limit by 2 so you shouldn't need as many houses.

Once you've progressed in the game and mined some iron build a forge and you'll have the option of creating a relic. Select the Mask of Gullinbursti which, like all relics costs 10 iron, 100 gold and 50 wood and once it's been made (it takes a while) you need to place it somewhere in your lands to pop the achievement.