Art: Great front panel and character art. Interior is perhaps disappointing in comparison, but functional.
Writing: Solidly thematic. Maybe just a little haphazard in how key details are sprinkled throughout. Everything seems addressed to the players, but it's the Warden who'll be reading the pamphlet.
Game Design: Love the nonlinearity of the plot and the ability to solve the problem multiple ways. I suspect that no two playthroughs are going to go exactly the same, which is what you want.
Theme: It's very Mothership and has a strong theme... I'm not sure that theme specifically comes across as "Ancient Mythology" to me, aside from mentioning that the players have "heard stories" about the monster.
Layout: I think the way the interior spread combines narrative timeline with spatial map is efficient. There are some minor alignment issues, especially in the encounter table where the items are inconsistently top-, middle-, or bottom-aligned in their boxes.
Utility: I felt a little confused and overwhelmed reading through this, although I think all the necessary information is in there somewhere. It might go a long way to summarize the NPCs in the Warden Notes. E.g. "Inside, players will encounter three surviving members of the Threshold's crew: Vivienne, who remains in stasis; Pell, who has gone mad; and Emil, who has become one with the beast."
Favorability: I could see myself running this as a one-shot.