[The following was scribbled in after the last note, in different ink]:

I found it! Finally, a description, though from some time after Saxo's death. This comes from a thaumatic, or wizard, in 278 prior, snugly in the middle of the Thaumaturgical Revolution. I had to piece together various fragments to read it properly and alas, there is no name left in the remnants, though there is a piece of the fellow's description of himself: "a gentleman of sightly stature and bearing, whose physical appointments are but augments to [his] mental prowess...." It is ironic vengeance that such a pillar of his age should be forgotten and his work torn asunder. I am certain the work was not written by one of the Greats of that time, but it is exactly what I was looking for: a concise definition of the Netherwild. Of course, it cannot be all encompassing, but it will do:

"When men speak of "the Netherwild," most often they are referring to an unstable, geographical phenomenon that has been reported, at one time or another throughout the written record, in nearly every square mile of Isumbras. In other circumastances, the wielder of the term might be referring to the supposed entity or entities that has been long supposed to rule or control the said phenomenon. In order to describe this peculiar environ, we will be greatly aided in preparing first an analogy."