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 Holy Church.
The nation is led by Pope Emperor Louis Valentin, who, secretly is a Vampire and subordinate of Demon Lord Luminous Valentine -- the "God" of Luminism, and the true ruler of Ruberios. Until his death, Roy Valentin, the twin brother of Louis, played the role of the "evil Demon Lord" Valentin to unite Ruberios against a supposed ever-present threat.
To gain military strength and raise morale amongst the populace, the nation founded the Holy Knight Order, 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.
Ruberios' founding roots can be traced back to over 2,000 years ago, when the Demon Lord, Luminous Valentine, lost her original vampire nation, the Nightrose, to Veldora's many random and destructive rampages. This event forced her to go into hiding, consolidate power, and build a brand new nation from the ground up when the opportunity arose.
Eventually, that chance came during the semi-millennial Tenma Great War -- an event where an army of Angels would descend from Heaven and reign destruction upon the world, to limit human society from advancing too far. Sensing an opportunity, Luminous took full advantage of the ensuing chaos to establish her own faith, propping herself up as a goddess. Thanks to the immense power she possessed as a True Demon Lord, she was easily able to attract followers. This new religion was quickly taken up by the human survivors who flocked to her en masse, eventually planting the roots that would begin its transformation into an Empire.
Underground Empire (Night Garden)
Not content with only having a single human empire and learning from the mistakes of her first country, Luminous decided she would also create a powerful underground empire within the belly of her human one, to sustain her non-human subjects. With the emergence of the Holy Empire of Ruberios, the secret underground empire inhabited by her fellow vampires would be sustained and maintained by her human populace.
This was done in order to save the endangered vampire race, whose numbers significantly dwindled due to the decrease in humans as a result of the Tenma War and Veldora's frequent rampages, effectively decreasing the overall "food supply" of the vampire race. While not a great threat to higher ranked vampires who have evolved past the need to rely on blood for sustenance, this development posed a severe existential threat to the lower ranked ones who would have starved to death unless something changed. In order to save both her subjects and humanity, Luminous successfully created a system within the Empire whereby her servants could live in peace in secrecy, and are provided with a stable source of human blood. Luminous herself lives in secrecy within the nation's borders and the Empire's emperor being her servant that act as her stand in when dealing with the supposed human centric empire.
Holy Empire Ruberios is noted to be one of the most powerful military powers in the world, with their overall military abilities earning them the attention of the Eastern Empire as one of its primary targets in their path of conquest.
Capital City Rune
The Capital City of the Holy Empire, noted to be one of the most well developed human cities ever built, is outfitted with high end magic and technology to maintain its prosperity.
A powerful gigantic city-wide barrier is stated to enclose the entire capital city, completely shielding the land from any external threats such as monsters, spirits, and demons that try to enter without permission. It is even strong enough to repel any magical based attack from the outside and completely block out or filter sunlight, if needed. Thanks to this, the climate and weather within the City proper can be manipulated at will and is primarily used by the Capital's residents to grow various vegetables and fruits not native to the country or even the continent, due to the needed conditions being naturally non-existent. They are also 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 Mountain is Luminous Valentine's primary base and home residence where she lives in secret away from her Human Empire. The mountain itself is well protected by both the Ruberios Holy Church and the Holy Empire's forces as it is considered their "holiest site" and is also secretly guarded by Luminous' powerful vampire servants.
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 mountain itself was specifically created and fortified to repel the likes of Storm Dragon Veldora and other beings of similar power to him.
The current (as of Volume 9) official relationship between Tempest and Ruberios is neutral at best, Rimuru and Luminas Valentine agreed on a non-intervention pact between each other and simultaneously their respective nations. Beneath the surface, they have cultural and technological exchanges.
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Due to wanting to remove Guy's power base and desiring to acquire the magic developed by the Western Holy Church, the Eastern Empire has placed it as their first target for invasion once they cross the Jura Forest and them even sending in advance scouts to spy on the empire in anticipation to their eventual invasion.
Ruberios in turn, is aware of them being targeted, and have built up their defenses for the Eastern Empire's invasion.
Due to the powerful influence of the Western Holy Church and their power base being primarily based in the Holy Empire Ruberios, the Council is known to be nothing more than puppets for the Empire due to the independent kingdom's that make up the Council being reliant on the power and support of the Empire to maintain public order via faith, healing and medical assistance from their local church, and providing protection from monster's through the paladins. Due to this, the Holy Empire can typically force the Western Council to go along with whatever they demand of them.