This is one of the most frequent questions I've been asked since I published Revelations.
As I was writing the book, I wanted an overall term for people with supernatural abilities. I tried a few variants I'd seen elsewhere: supes and SPPs (super-powered people/person). I toyed with calling them some variant of demigods (based on the ancient myths of gods mating with humans to create gifted individuals) but I couldn't find something that worked.
I'd already established Dani in my mind as one of the line of Babylon priestesses, living in hiding and exile for thousands of years. The main language in Babylon was Akkadian so I decided to hit Google and see if I could find an Akkadian word which might match what I wanted.
I found a list of about thirty terms used in Akkadian (but originally Sumerian), one of which was lalassu, defined as spectre. Spectre, ghost, hidden ... the ghostly people. That's how I came to use lalassu as my overall term for people in a secret society of superpowered people.