Here is the full list of all 31 Ortus Regni achievements.
Finish the tutorial
Become King
Decide the Battle Outcome as a result of the Church Decides Card
Enter the 2P Leaderboards
Win a game without playing any Castles
Be on the winning side of a Battle Outcome
Assassinate a Lord with Treachery
Hold a Joust
Take control of the Vikings
Block a Political Attack with an Allies Card
Capture a Mercenary using an Intrigue Card
Enter the 3P Leaderboards
Enter the 4P Leaderboards
Perform an Abbot Reveal
Win a game using a deck with 6 Banquets
Win a game using a deck without any Banners
Play a Palace Card from your Hand
Be in a Political Struggle that involved 3 Vassals
Deal 20 damage in one turn
Possess a Fief with 8 Properties
Possess 10 Viking Influence Cubes
Bequeath 3 times in a single game
Win 3 jousts in a single game
Win a game using a deck without any Vassals or Allies
Win a game using a deck with 6 Banners and 6 Princes
Win a game using a deck with 6 Mercenaries and 6 Castles
Have 10 Friends
Win 100 games
Destroy 50 Fiefs
Recruit 80 Armies
Tower 60 cards