Northgard

This relic belongs to the Wolf clan so start a new game, pick the wolf clan and play as normal. They gain food when killing the wildlife (wolves and bears) and their soldiers eat less food so this is a war clan.

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 Horn of Managarm which 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. This relic let's you summon up to 3 White Wolves that you can control in battle.