"Forgotten PS1 horror game" is certainly a good vibe and the Reach is a yet another good example of this.
Presentation is excellent. It looks exactly like an old and worn manual or guidebook from the era, completed by faded screenshots of low poly environments, magic items and the False men. A welcome side effect of using this style is that the layout is easy to follow and read through. The writing is also on point as you've managed present a mystery that players and GMs would love to delve into; the Reach isn't just a network of caves, its alive and alien in nature. I feel like the Reach would work just as well as a stand alone setting as it would as an expansion for the wretched world of Mork Borg.
I will say I would prefer some less abstract stats for the False Men. While stat bars are part of the PS1 manual aesthetic it could prove difficult for a GM to know how much HP a NPC has or how much damage a monster does using stat bars.
The only other criticism I could add is that I wish there was more, but I would argue that's also the highest praise I can give the entry. I would love to see more of the Reach, I would love to see what wares the Nether Merchants offer, to hear what the Whispering Traps say, and to uncover what horrors, wonders and mysteries lie closer to its heart. I agree with the previous comments; I would love to see a zine of the Reach.