The First Generation Demon Lords are the oldest Demon Lords in the history of the world. Historically, they have close connections with the Creator, and have played huge roles in the shaping of the current world. Chronologically, they were Guy Crimson, Milim Nava and then Ramiris. They are the only demon lords that managed to keep their seats at the top even after all the wars that took place.
Guy is first Demon to ever come into existence. Being the oldest Demon, he has been summoned and used countless times and has battled against countless enemies, until he reached the stage of an Arch Demon, the pinnacle of the Demon race's natural evolution chain. It is also during this time that he had possibly obtained the never-mentioned Unique Skill Pride. Being a Demon, he longed for more strength and more freedom to rule however he wanted. Thus, after he was summoned once again, he decided it would be his last.
He accepted his summoner's wish to eradicate an enemy nation, but he took the lives of his home nation as compensation, thus ascending him into a Demon Lord and granting him the Ultimate Skill Prideful King Lucifer. This was also when he evolved into a Demon Duke, the first existence of its kind and also the very first Demon Lord to come into existence. Following after, he summoned two Arch Demons, and sent them to fulfill the contract of eradicating the enemy nation.
Ramiris is the only one of the First Gens who is not a True Demon Lord, meaning she is a self-proclaimed one. Guy and Ramiris having the history and connection of being chosen ones by Veldanava possibly decided the Demon Lords to be an organized force to maintain a certain degree of order and represent a symbol of fear, especially since the true maintainer of order was no longer here. It is also possible the organized force of Demon Lords came into existence to protect and/or control Milim, since Ramiris and Guy shared the connection of being the ones simmer down the berserk Milim.