This idea came while listening to a song on the radio (I later found out it is called Welcome to the Black Parade by My Chemical Romance). It is not particularly original, as I mostly attempted to make the lyrics at the beginning of the song make sense literally. I just now combined it with another idea I had after reading reports from the Realms Beyond Adventure: Boudica the Great for Civilization IV.
The opening of the song:
When I was a young boy,
My father took me into the city
To see a marching band.
"Son when you grow up, will you be the savior of the broken,
The beaten and the damned?"
"Will you defeat them, your demons, and all the non believers, the plans that they have made?"
Because one day I leave you,
A phantom to lead you in the summer,
To join the black parade."
In my head, the marching band was not made up of musicians but the unique warriors of a reviled but misunderstood nation. These warriors act as hosts for phantoms, disembodied spirits that grant fantastic powers. They render their hosts ageless and effectively immortal barring some accident. Unfortunately, the phantoms also utilize their hosts to wage and unending war on demonkind, so often do they come face to face with fatal accidents.
The phantoms live on unscathed by the death of the host. I have not determined how sentient and conscious the host is in the partnership. Does he yield both body and will to the phantom, effectively ceasing to be? Or are they partners in a symbiotic relationship, each offering their strengths to the other?
Regardless, these phantoms and their hosts gather each summer in the capital of the nation and create the Black Parade. They march throughout the entire land when demonkind is at its weakest and least active, bolstering the will of the people. The citizens of this nation know the immortal phantoms by sight, glimpsing them in two ways. First, much like the spirit of wrath that possessed Orson in Record of Lodoss War: Chronicles of a Heroic Knight, the phantoms loom transparent above their hosts. Then, also like that spirit, they burrow their way inside the body and elicit a physical change. Where the spirit of wrath caused Orson's muscles to bulge and his eyes to roll back until only the whites showed, these phantoms can cause drastically more extreme changes. Some hosts expand to 10 feet in height while others grow multiple sets of arms and sprout horns. All of them adopt fanciful coloration (not necessarily colorful; even a harlequin is fanciful in but black and white) and accouterments. These forms are unique to each phantom, and thus they relay at each year's Black Parade that while the human hosts may differ, the ever vigilant phantoms remain the same.
How does this combine with Civilization IV? In any game of Civilization, you play as a particular leader, such as Boudica of the Celts or Isabella of Spain. But the game spans more than 6000 years of time, so how is it that the same leader traits characterize your civilization's leaders throughout that entire period? Obviously it is an abstraction in the game, but the concept of the phantoms allows for that to be true.
This nation is ruled by a phantom who possessed one host after another from time immemorial. All of its hosts in the recent past (several generations) have been willing, allowing themselves to be possessed in service of their countrymen. Whether this was true in the past is unknown. I will note here as well that the phantom population at large possesses willing hosts, often with a form of heredity mixed in (such as the singer receiving the phantom from his father; this would also imply a level of symbiosis between host and phantom rather than the phantom completely taking over the host's body and quashing his will).
Enough for now, time for bed.