Twilight of the Gods
Denmark was the first Norse country to become a monarchy, but like all Norse monarchy, the authority of a given King is highly dependent on his own charisma and military successes. There is also no clear system of inheritance, meaning multiple sons can and do fight each other for their father’s throne when he dies.
To the north Denmark is open to the sea. To the south, Denmark frequently contends with the remnants of the Christian empire built by Charlemagne.
Like Norway, Denmark is slowly being Christianized. Kings hoping to expand their power find that the way of the White Christ, and monotheism, is much more effective for making sure authority rests with one person.
The last king of Denmark, Harald the “Bluetooth”, conquered part of Norway as well, and Ollaf of Norway is in fact his son. Ollaf’s seat of authority remains in Norway, but Denmark is technically under his control. The Jarls and Thanes of that land must cross the sea to swear to him.