Go to Val Royeaux. Search for a vendor named "Deraboam" located in a small store with a blue door. Interact with the chest there to find "The Mystery Box". Spend 10,000 coins to purchase it and get the "The Big One?" mission to appear on the global war board. Send one of your advisors on the "The Big One?" mission and they will return with a giant Nug mount as the reward.