Twelve Guardian Lords

"Since these twelve people now had lord epithets, I decided to refer to them collectively as the Twelve Guardian Lords. Some of them were also part of groups like the Big Four and the Ten Dungeon Marvels, but the lord term in their titles took precedence. That’s because, unlike those two other groups, I had no plans to replace these twelve anytime soon. Being a lord, in my realm, is a sort of lifelong appointment, since all of them had eternal life in the first place. In the future, I think it’d be ideal if they eventually stepped away from day-to-day duties and only took up their Lordly roles during wars or emergencies."

- Rimuru

In Tempest, the title "Lord" was established to signify lifelong appointments of those with eternal life spans, power equivalent to a True Demon Lord, and as a reward for exceptional service. Following Tempest's victory over the Eastern Empire's Armored Division, Rimuru granted this honor to twelve of his subordinates, and they became known as the Twelve Guardian Lords (聖魔十二守護王).

Background
During Tempest's first battle against the Empire's Armored Division, Raphael discovered that it can use Rimuru's Ultimate Skill Beelzebuth to convert human souls into energy, and use them to evolve qualified majin and daemon subordinates to True Demon Lords for the cost of 100,000 souls each.

During the conflict, Rimuru collected 940,000 souls and, since the souls of particularly strong individuals such as Enlightened and Saints had more energy than usual, he was able to normalize that value to the equivalent of over 1 million souls - allowing him to evolve up to 10 individuals to True Demon Lords at that time. An important consideration, however, is that although these people would evolve to "True" Demon Lords, it wouldn't be appropriate for Rimuru to grant them the title "Demon Lord" which would put them on equal footing as himself. Instead, he granted each of those equivalent to True Demon Lords their own unique title ending in Lord, a sort of Demon Lord in his service. This also provided an additional reward in the form of the title, and distinguished them as immortal and permanent members of government. He did not evolve the Three Devil Girls at this time, but found it appropriate to give them Lord titles as well as they already met those qualifications.

During the final battle against the Empire’s forces he evolved the trio as well, using the souls he acquired from Guy Crimson.

Trivia

 * The titles granted to Testarossa, Ultima, and Carrera were based on recommendations from their subordinates, while the others were solely Rimuru's idea.
 * Carrera has the highest amount in terms of Existence Value among all the Guardians.