I’ll admit, I read this word in a novel and I couldn’t remember what the word meant. So, being curious as I am, I went in search of a definition. After not finding the word in the small dictionary I was able to leave unpacked, I kept searching and I found the following definition on the Urban Dictionary site.
Obsidious: Besieging; besetting, siege like, besiege, unrelenting, attempt to overpower, an extensive period of misery.
Now if I could just find the word again in the book I can see which one of these connotations is correct for the passage. I’m thinking “unrelenting” is probably the right one.