The Argurons are members of a secret subterranean civilization beneath Paris (apparently, one of many) known as the Silver Lands. They seem to be human, but some of them seen in the Arguron palace are than Ardsley Wooster and have so little pigmentation that they could be albinos, almost as if they are a separate race.
When we first meet them, the Argurons are ruled by an unnamed but very.. enthusiastic.. King who was originally a surface-dweller who evidently hit it off with the late Queen and married into the royal family. (It's an old story..) Judging from the appearance of his daughter, Princess Larana Chroma, the queen was one of the tall pale Argurons.
The Argurons have also been locked in a generations-long war with their (very) close neighbors, the Talpini Moligarchy, but recently have reached an accord with these traditional enemies, due to the efforts of Jiminez Hoffmann, who arranged a political marriage between members of the Arguron and Talpini Royal Families. (The latter being himself, having been officially adopted by the Talpini king.)
When Agatha and Co. visit the palace with her Wasp-Eater in tow, it is revealed that many, if not all, Argurons are Revenants. The King additionally reveals that he is in league with "a Glorious Queen," who seems to be the Other. Larana disagrees vehemently with this course for the kingdom, and rebels against her father, agreeing to go along with the aforementioned marriage plan to secure the Talpinis' aid, not knowing that it is her crush Jiminez she's agreed to marry. In the end, she overthrows her father and becomes Queen, ushering in a new era of cooperation among the various under-Paris kingdoms; we later hear that she and Jiminez do get married.
As far as we know the name "Silver Lands" has nothing much to do with the Citadel of Silver Light.