Dominion (Card Game) Wiki
Type Victory
Effect Worth 1 VP per Duchy you have.
Cost 5 Coins
Set Intrigue
Illustration Christof Tisch

Additional Rules from the Rulebook[]

This does nothing until the end of the game, at which time it's worth 1 per Duchy you have. This counts all of your cards - your Discard pile and hand are part of your Deck at that point. During set-up, place 12 Dukes in the Supply for a 3- or 4- player game and 8 in the Supply for a 2-player game.

The Secret History of Duke[]

In an article on Board Game Geek, Donald X. Vaccarino commented on the development of Duke:

Another card added when the set went to 25 cards. I picked this particular card for this set to round out the victory card theme. Baron makes Estates matter, Duke makes Duchies matter.
This card started its life in another set as a victory card worth one point per copy of it you had. That just varied way too much. You have to charge enough for it to be fair when you get a lot of them, and then it's horrible when you don't. The fix was to have it count another card. The other card had to be one that was always out, and Duchy was the obvious choice.[1]