[Perhaps I will address these topics more thematically than I had originally planned, for as I set about my library for a place to start, beyond those mere scraps that were the beginning of this all, I came to the first books I owned of my own accord. Certainly, at the palace there was an unparalleled library, but in the wild, so to speak, books were, for a time, difficult to come by. It is appropriate that those first volumes, which I procured from a strange old man in the grim foothills of the Frostgate, make up a nearly-complete set of Saxo Merkado's The Golden Horn: A Geographical and Occasional Cultural Survey, ca. 147 MR. It was to these that I first turned when Nicodemus left my door, and thus it seems fitting that it is to them again that I turn in this new undertaking]:

"From what I can gather, the common usage of the term Isumbras is far from its original intent. Today, when you or I refer to Isumbras, we are of course discussing the entire region, from the Frostgate to the Southern Sentinels, from the ocean to the Skybiters, and everything contained therein.

"As has been likely passed from parent to child through countless tales and lore, in every village and homestead, the word "Isumbras" means simply "The Golden Horn." Of course, the origin of such a name is described in as many tales and pieces of lore, each of them with their own national or cultural flavor. In a trading post in Cilan, I heard a story about a hunter in the days when the demons were men who sought a wild bull with golden blood and a single horn, which was its only weakness. The men there said that the hunter's attempts to capture the bull, and the animal's subsequent attempts to escape led to the formation of the border mountains, as well as many other geographical features."
I haven't been able to discover where this story came from. It is not, as far as I can tell, a part of any of the usual Cilanese traditions, nor does it appear to have Narti roots. It must have been a very local tradition, perhaps a modification of the aboriginal hornless bull of the harvest, which ironically makes it closer to the truth of the origin of the name Isumbras than one might think.

"The truth of the name is found in the roots of the word itself, not surprisingly. Isumbras comes from the gipsy words is and ras which do, literally, mean 'gold' and 'horn.' But they mean much more in the context in which they were originally coined. Gold, in this case, means something really more like 'rich,' or 'valuable,' or even 'abundant,' while this particular usage of 'horn' refers to an old gipsy legend about a horn from a ram that kept its starving master alive without giving up its own flesh (and thus saving its own life) by pulling off one of its horns, from which continually spilled food and drink of all kinds.

"The beginning usage of the word, then, in relation to the known world is, sadly, quite mundane. The gipsies spoke of the Gold Horn, namely the land close to the shores of the Irisidesian Sea, which, if one looks at a map, resembles somewhat the horn of a ram, indicating both its shape, and its abundance of sustenance and life. It was not until much later, after the Migrations of course, that the term began to apply to everything from Cilan to Castille."
Only the gipsies, who were immune to the corruption effects of the Netherwild, could refer to that region as an "abundance of sustenance and life" without jest.