Holy Empire Ruberios 「神聖法皇国 ルベリオス, Shinsei Hououkoku Ruberiosu」 is the second largest and most powerful Kingdom in the western continent.
[Translations write it as "Ruberion" and not "Ruberios" nor "Ruberius". Dunno why. In the very latest chapters i.e. Arc 10 of the Web Novel, it is translated correctly as "Ruberios."]
Demon Lord Ruminas wasn't interested in simply hiding, and built an underground empire. And on the top, she created an empire to sustain her underground empire.
Due to poverty caused by the conflict between angels and demons, there were few mighty nations. And the Eastern Empire had barely taken form. They couldn't even send forces to protect the people from the threat of monsters. Naturally, almost everyone was a refugee or a bandit, and the world was even more turmoil.
Precisely because it was such a time that Ruminas, with the power of a demon lord, was able to control monsters and easily control humans. In the name of salvation, humans came to her and she granted mercy to those who gathered under her. Using blood (Life) as bait, she granted them protection and the country was formed in this way. The Holy Kingdom Ruberion was established by Ruminas, with her being the Emperor (Empress?). As that country was born it would've inevitably be controlled by some ambitious or greedy man—the cause of Ruminas’ worry. So, Gilles, as her right hand, acted as the pope. However, this fact was only known by a few selected people in the kingdom.
After the resurrection of Hinata, Ruminas decided to have the Kingdom and the Church ally with Tempest in order to rescue The Hero from Yuuki's control.