Why are the Black Serpents in the Chasm underground mines?

Hello, i wanna say that i havent started the Archon quest in the chasm yet, just doing the world quest rn, so if this questions gets answered there im sorry.

In general i find some parts of the lore really complicated especially because my english is not really good.

So after reaching the underground mines I met some new enemies, the black serpents. I learned that they are members of Khaenrias royal guard. Because of that I was first like, wait, does that mean that Khaenriah was located under the chasm? After checking i figured out that this is not the case and that another, older ancient civilization was living there, probably the same civilization as the one from enkanomiya? (I havent been there yet)

Now what I would like to know is, why are those royal guards even there? What is the connection between Khaenriah and the chasm/ the chasms old civilization? And finally, what actually happened to the ancient civilization living under the chasm, why are they gone?