Monumental

The honest route is building a duchy building in your duchy capital and paying through every upgrade tier - years and a small fortune. The wiki's delightful shortcut: create a faith with the Megalithic Constructions tenet, whose unique duchy building (the Great Megalith) has no upgrade tiers - so it counts as "fully upgraded" the moment it finishes, popping the achievement on completion. You must personally hold both the duchy title and the county. Creating the faith en route is It's not a Cult! anyway.