Holy Empire Ruberios (神聖法皇国ルベリオス shinsei hōōkoku ruberiosu, lit. "Holy Papal Empire Ruberios") is the theocratic headquarters of the Western Saints Church.
The nation is led by Pope Emperor Louis Valentine, but he is actually a subordinate Vampire of Demon Lord Luminous Valentine, whose first name is used as the God of the Western Nations' greatest religion and who rules Ruberios from the shadows. Until his death, Roy Valentine, who was Louis' twin brother, played the strawman role of the "villainous Demon Lord" Valentine to unite Ruberios against a supposed ever-present threat.
The Holy Empire, to gain military strength, and raise morale and sense of partial security amongst the people, created the Holy Knights, which often consisted of otherworlders, such as Hinata Sakaguchi. These knights were recognized as a powerful military force across the world of Tensei Shitara Slime Datta Ken.
The Founding of the Holy Empire Ruberios was achieved when the Demon Lord, Luminous Valentine, lost her original vampire nation, The Night Rose, to one of the Storm Dragon Veldora's many random and destructive rampages which completely annihilated her first nation. Forcing her to go into hiding, consolidate power, and build a brand new nation when the opportunity arose.
Eventually, the chance came during the annual Tenma Great War, where the gates of Heaven would be opened and the Angels would begin their destructive rampage across the world to limit and suppress human society from advancing to much and keep the world stable. Resulting in wide-scale human death, destruction of nations and empires and the complete chaos in the world at large. Seeing the opportunity, Luminous took full advantage of this chaos to found her own faith that deified her as a goddess, A claim she easily backed up thanks to the immense power she possessed as a True Demon Lord. This new religion was immediately taken up by the human survivors who began flocking to her in mass, eventually setting the roots that would begin its transformation into an Empire.
Not content with simply having a single human empire and learning from the mistakes of her first country, Luminous decided that she would also create a powerful underground empire within the belly of her human one that would sustain her non-human subjects. With the emergence of the Holy Empire of Ruberios, Luminous had also created a secret underground empire of her fellow vampires that was thus then sustained and maintained by her human empire. This was done in order to save the endangered vampire race, who due to the decrease in humanity's population due to the Tenma Great War and Veldora's frequent rampages, severely shrinked their numbers. Effectively decreasing the overall "food supply" of the vampire race, that while not a great threat to higher ranked vampires who have evolved past the need to drink blood to sustain themselves, this development posed a severe substantial threat to the lower ranked ones who would have begun starving to death unless something changed. In order to save both her subjects and humanity, Luminous successfully created a system within the Empire where they were successfully provided a stable location where her servants could successfully live in peace safety in hiding and provided with a stable source of human blood to feed them. With several of the Empire's highest ranking officials and confidants being in fact, Luminous's vampire servants hiding in plain sight. Luminous herself lives in secrecy within the nation's borders and the Empire's emperor being her servants that act as her stand in when dealing with the supposed human centric empire.
Western Saint Church
The Holy Empire Ruberios is noted to be one of the most powerful military powers within the world, with their overall military abilities making them one of the Sovereign Alliance of the Nasca Namrium Ulmeria Empire primary targets once their plans of expansion was initiated in order to both eliminate a major power from their path of conquest as well as assimilate the Holy Empire's military strength into their own nation.
The Empire's Holy Church in particular is noted to be the central most core of their military power and the foundation of their nation's morals and justifications. As seen where the church's paladins are one's charged with the duty to eliminate dangerous monsters plaguing their nation as well as allied nations and fighting off demon's and demon lords when they appear. The Holy Church is also the ones responsible for the large scale rituals in order to summon Otherworlders for the sake of further military expansion and training a new "Hero" to further give hope and moral to the relatively weak human race.
Within the Church's forces, they have the Ten Great Saints, who are the pinnacle of their soldiers, magic users, and heroes that even counts the infamous Hinata Sakaguchi as one of their leaders. And even higher up to them there are the semi-immortal Seven Celestial Sages who are charged with training them and the True Hero to further strengthen humanity.
Unknown to the human inhabitants, The Holy Empire Ruberios also possess access to the forces of the True Demon Lord, Luminous Valentine's Vampire Servants and her other Demi-Human servants that lived in secret within their nation that can further bolster their military power and resources during times of crisis.
This in turn acted as a substantial deterrent against any foreign power that tried to invade Ruberios due to Luminous and her powerful servants presence within their nation.
Other than the Paladins of the Holy Church and the secret non-human forces of Luminous Valentine, the Ruberios Empire also has a common army made from simple foot soldiers enlisted from the general populace that primarily act as foot soldiers for the Paladins and Knights of the Holy Church and are generally given the duty of maintaining the internal order within the Empire's cities, villages and towns. During times of war, the Paladins of the Holy Church have the authority to assume command of them as their soldiers.
The Capital City of the Holy Ruberios Empire was noted to be one of the most well developed human city's ever built and out-fitted with high end magics and technology to maintain its prosperity.
A powerful gigantic city-wide barrier was stated to enclose the entire capital city that completely shielded the city from any external threats such as monsters, spirits, and demon that try to enter without permission and was even stated to be strong enough to repel any magical based attack from the outside. It was even stated that the Barrier itself was strong enough to completely block out or filter sunlight if needed. Thanks to this the climate and weather within the City's limits is actually manipulable and is primarily used by the Capital's residents to grow various vegetables and fruits usually not found within their country or even the continent due the needed climates to make them grow not native to their country and even affect the growth of their own food production to increase further due to now being able to grow and harvest any type of food even if off season. This in turn generates a great deal of revenue for the Empire's economy and insures that no one within the Capital goes hungry.
The Sacred Hill is Luminous Valentine's primary base and home residence where she lives in secret away from her Human Empire. The Hill itself is well protected by both the Ruberios Holy Church due to it being considered their "holiest site" and by Luminous's powerful vampire servants. Later as noted by Hinata Sakaguchi the Palace itself was built to be incredibly sturdy and well fortified to the point that not even the most powerful magical attacks can pierce it from the outside. It was later revealed that the Hill itself was created to repel Storm Dragon, Veldora Tempest and other beings of similar power to him.