User blog comment:Evil-Tree/Theory: The world of Hilda works on the magic of True Names/@comment-4946399-20191223095148/@comment-2055627-20191223214005

Looking at how the marra ally themselves with the Rat King yet don't sign Elf contracts it seems they only engage in magic and other magical creatures if it helps with their nightmare making and may be ignorant of this. That could really come back to bite them if something/one were to take advantage of that. (Not gonna lie, I've been thinking of a fanfic plot of something along these lines)

As for Trundle and beings like Jorgen it could be either they hide their true names and use these as an alias or they are simply so ancient and powerful they have no need to hide their true names.