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 knight were recognised as a powerful military force across the world of Tensei Shitara Slime Datta Ken.