His epithet in Japanese is
Meio, which also means Hades. Probably refers to his lethality or implies that fighting him is one way to be sent to hell.
Translation: [
Dark King] Those who chose to face him only win a one way ticket to the underworld. He may be old but that's how tremendously powerful he is.
I guess "Dark King" was a translation issue- it probably should be King of The Underworld or something similar, maybe?
Personally, my initial assumption before I had learned of this was that he got the name because he had the power to be a Yonko or challenge Roger, but willingly didn't.
Hence "Dark King" as in one who had a "king's" quality and power, but willingly chose to be in "the dark" of another's shadow.