Dice Poker: The Legend is not an official quest, it is the last phase and ultimate goal of A Game of Dice. It is also independent of the other three dice poker sub-quests. Here, the stakes are the highest, but the title of "legend" and 1000 XP hang in the balance.
Well, you have played and beaten everyone you know at dice poker. Can anyone beat you? It just so happens that King Foltest likes to gamble a little and is pretty good at it. If only he had a worthy opponent who was not afraid of being executed for beating him...
Do not worry about finding him, he will find you. Play poker with him when given the opportunity (at the beginning of Chapter five, when you speak with the king after he summons you to his chambers). You won't get another chance.