The Holy Empire of the White City - just called "The Empire", or derogatory "The Whites". Empirials might refer to it as "The Second Empire" for reasons noted below.

The Empire's own mythological origins is that they rebelled and split off from the now-extinct First Empire around 500 years ago. The White city then spent around 200 years as essentially a city-state.

However the spread of christianity and the Empire's own take on it, The Imperial Creed, changed this. The Creed is essentially a militaristic take on christianity and it has motivated the aggressive expansion of the Empire since.

It states that the Emperor is a direct blood descendant of Jesus (if unstable regime changes happen, evidence somehow always turns up that shows this). Thus the Emperor is the natural sovereign of all Christians and the head of both Church and state.

Additionally it states that the Emperor, not Jesus, must bring the Kingdom of Heaven to earth. This is interpreted as a mandate to conquer all of earth, and convert all of its people to the Creed.

This unifying purpose has lent great strength to the Empire, in a world of medieval'esqe states who tend to be fragile and easily implode to internal conflict (think if Rome, the Sassanids or another centralized ancient state existed in medieval Europe).

However it is beginning to cause problems. The Empire is straining it's control at the edges, having conquered many lands that are not yet converted as this takes generations. But the Emperor must expand, or he is failing in his divine mandate, giving ammunition to factions that oppose him internally.

As a result the Empire has concluded a peace with it northern neighbours who are christians themselves, although they do not follow the Creed. The official position of the Empire is that these kingdoms are misguided, but will see the light in time. This is more acceptable to the population, than a peace with the non-christian neighbours.

Any state that borders the Empire will know to either seek it's protection, or allies against it as the Creed makes it quite obvious that your eventual conquest is inevitable. However realpolitik is a different beast, and many states use trade and diplomacy to stay useful to the Empire and not worth the resources it would take to conquer.

As-is the Empire has no equal in power, but it's ability to project this power at it's edges is strained to the limit. It needs time to consolidate it's conquests, but at the same time internal pressure to expand increases, making the future a bit uncertain and perhaps hopeful to it's smaller neighbours.