Crusader Kings III

This is obtained by choosing the 'Champion the Faith of the Country Basques' major decision. The requirements are 1000 Prestige and 1000 Piety and having at least 9 counties in the correct area of the map.

This is very easy when creating a custom character as the King of France (West Frankia) in 867. This is due to you having enough counties already for the decision. Do not give any counties away even if you are above your maximum. Keep Catholicism as your religion and choose the Basque culture (option under 'Iberian'). Bring your age down to 16 so you can use the most customization points for Personality options and Other Traits options that increase your prestige and piety gains. For Personality, I choose Cynical and Temperate. For Other Traits, I choose August, Gallant, Theologian, Pilgrim, and Raider. I choose Insightful Thinker for Education and then removed Prowess points until I was at 400. Then make sure Ironman mode is enabled so achievements are valid

After starting the game, choose to go on a Pilgrimage right away and select any option that gains Prestige. Otherwise choose an option that gains Piety. After that, save your gold until you can Call a Hunt and take the Prestige gains. You should be very close to 1000 of each after this and just skip ahead in time until you do. Once you have 1000 Prestige and Piety, choose the decision, select any of the top three options, resume time and achievement will unlock.