One of the more amusing ideas we get from the Middle-Ages is the concept that the wellbeing of a people or nation is directly tied to the wellbeing of the leader or monarch. So if the queen of a country is happy and healthy, the land, people, animals, etc. will be happy and healthy too.
You can see this in various works of the past and present. For instance, the Arthurian legends contain this idea. While things are right among Arthur, his wife, and his knights, the kingdom prospers. When Lancelot and Gwenevere disrupt that balance, Camelot plunges into darkness.
Also, you can see this idea represented in Disney's "The Lion King" animated movie. While Mufasa is in charge, the Pridelands are green and full of life. Mufasa is happy, wise, and in full vigor. When the unhappy, restless, weak, and greedy Scar takes over, the Pridelands decay into a wasteland.
If I were to use this idea, I think I'd want something other than an unexplained mystical tie between king and country to be the cause. For instance, the link could be established by some magic item or an enchantment placed by a powerful Ancient being could be the cause.
Perhaps the item or enchantment could then be disrupted by evil in some way, setting the characters on a quest to make it right. They would have to recover the item or find an Ancient capable of recasting the spell. This tweak keeps the romanticism of the old beliefs but adds the plausibility the modern mind demands.