Diablo was already a "Daemon Lord (悪魔公(デーモンロード) dēmonrōdo, lit. "Demon Peer")" before he was named by Rimuru. That's something we learn in LN volume 7. It has nothing to do with him training and such (since as you said he doesn't care about raw power, and he doesn't have any high ambition for himself).
The rules for Daemons are different in the Light Novel.
It is possible to become normally evolve beyond Arch Daemon into a Daemon Lord if one lives for 2000 years while staying at the upmost magicule limit of Arch Daemon (meaning to never lose once, since that can cause them to lose Magicules).
Do note that "Daemon Lord"/"Demon Peer" is also sometimes translated in fan translations as "Demon Noble"(which includes Demon Duke from the web novel, and more).
That is to say that the Primordial Daemons aren't the only ones who have reached Daemon Lord class. Like I said in a previous comment, Moss (one of Testarossa's subordinates) is a Demon Duke in the LN(and is implied to be a Demon Noble/Peer class too). That's why the Primordial ones are also referred to as Daemon Kings/Queens, for they are beyond even the other daemons who are in the same species evolution class as themselves.
(Another thing about "Demon Noble": It seems that the term was originally coined to differentiate it from "Demon Lord" since the pronunciation characters (furigana) for the kanji characters of that term(Daemon Lord/Demon Noble/Demon Peer) sounds very close to saying "Demon Lord"(the generally agreed upon translation for maou).
"Daemon Lord" is a more recent translation of it based on two things:
1-The furigana in question, since the デーモンロード essentially has an elongated sound for the first character (デー).
2- The light novel illustration of the seven Primordials has the English word "Daemon" rather than "Demon" on it.
This version "Daemon Lord" is closer to the actual meaning of the term and it also easier to differentiate from "Demon Lord." So the wiki and the fan translation has started to use this version. (The person who popularized this version is one of the moderators of the wiki.)
@NovaXII The two daemons who came with Diablo are his subordinates (
it's confirmed in the novel, so Black Daemons, though one of them seems here to not have black hair, but that's likely a situation similar to Beretta's(also a Black Daemon) where the hair simply lost its color over time due to not maintaining high level of power and thus degrading a bit(but that does imply him being incredibly old
).
As for their power, it should be at least a bit more powerful than Beretta
(in virtue of them being Greater Daemons like Beretta(in plus of also being Black Daemons like him), while also taking into consideration that they are evaluated to be at the borderline crossover beyond the Greater Daemon class, as in that they could turn into Arch-Daemons in the near future(or that they were recently Arch-Daemons, but died enough and thus lost enough magicules that they just reverted to the top of Greater Daemon class)
.
Edit: It is also possible that Razen didn't evaluate their true power correctly. If so, they might actually be Arc-Daemons already (or maybe just one of them), and Razen simply didn't notice. Black Daemons have a tendency to be very eccentric and peculiar, and they usually tend to not seek power for the sake of power (instead just being whimsical and getting ridiculously invested in whatever interests them). So they don't always have a sort of "aura" of power around them. It's still a likely possibility that Razen just didn't perceive their real power and underestimated them like he did with Diablo/Noir (thinking him to be a Greater Daemon).
The reason Razen didn't just assume Diablo to be an Arch-Daemon is due to him having already killed an Arch-Daemon in the past, and he felt Diablo to be much more powerful.
only Noir aka Diablo is the only one called black daemon no one else. also diablo is just a really really old and powerful arc demon but his subordinate is just old greater demon
and diablo cannot normally evolve from arc demon because
all demons evolution past arc demon is locked and people need to gather all seven deadly sins ultimate skill series or have a demon lord seed and 10k souls like guy or forcefully evolve them by giving them 100k souls from rimuru's belzebuth
diablo is part of an eccentric family line but they isn't specifically called "black daemons"
@Andiza Clearly you're the one who doesn't know what he's talking about.
The Daemon Priomordials/Progenitors are known as such due to them being the seven original Daemons, the ones from whom all other Daemons originate. Each of them is the head of a different Daemon family, each family is identified by the color of its progenitor (and the demons in each family usually have hair of the same color as the family's Primordial, though in some cases the hair color can change,
such as in the cases of Beretta and the daemon with Diablo in this chapter who doesn't have black hair
).
Guy Crimson rules over Red, Green and Blue Daemons (since he's the Red Progenitor, and his maids are the Green and Blue Progenitors). Testarossa is the White Progenitor, Carrera is the Yellow Progenitor and Ultima is the Violet Progenitor. Each have their own Daemon family.
Diablo having been a Daemon Lord since before being named by Rimuru is something that's revealed in LN volume 7.
Also, the thing about Daemons not being able to evolve beyond Arc-Daemon is only the way you described it in the web novel. In the light novel, it's different.
It is possible to normally evolve beyond Arch Daemon into a Daemon Lord if one lives for 2000 years while staying at the upmost magicule limit of Arch Daemon (meaning to never lose once, since that can cause them to lose Magicules). As a result, not only the Primordials, but also a few of their old subordinates are also at that species evolution class (such as to-be-Moss(one of Testarossa/Blanc's top subordinates)).
Also, here's a spoiler I heard about from the later LN volumes (that haven't been fan translated yet):
Carrera by herself(while nameless) is considered a threat to Leon Cromwell's entire continent and country, and all of the Daemon Primordials (other than Guy Crimson) are apparently around that level too while they were still nameless.