Jairen, the only one who can defeat the evil king ruling the world of Shadowgate, is trained for the task by Hawk and Lakmir, in a story based on the Nintendo action-adventure game
F.X. Nine is a pseudonym used by Seth Godin for the Worlds of Power series. He was the creator of the series and wrote outlines for each book. Authors were hired to write the novelizations based on Godin's vision and outline.
This is part of the Worlds of Power series of books based on video games, which I remember seeing in stores when I was a kid, but they were mostly about games I wasn't familiar with. The same pseudonym is used for all of them, but it looks like the real author on this one was Ellen Miles. I finally played Shadowgate (the 2014 remake) last year, and I found out when looking up the backstory that Miles worked with some of the designers, so it's more accurate to the game than most of the series. It's presented as a prequel, detailing Jairen's adventure before coming to Castle Shadowgate, accompanied by a Fenling named Fezwyn Quickfoot. There's an explanation that all of the humanoids were descended from elves, but developed into different species depending on where they lived. Jairen meets up with the wizard Lakmir and several other allies, and fights off several sorts of monsters. The hero's backstory has been changed between different releases in the franchise, but some of the elements have remained basically consistent.
If I'd read this as a kid, I would have been allll over it. Just my cuppa. The prequel did a good job of setting the stage for the game; I guess the author couldn't get their head around writing the game itself and took this way out. Kind of a good idea, to be honest.