TL;DR: Liked it. Would of loves to have some more fleshed out NPCs. Missing a few connective bits.
I ran this module as a one-shot last weekend and we had a lot of fun! Like mentioned in the other comments, I did add a door between room 1 and 6. The players and myself liked the dark demonic and insect inspired themes.
I did have some feedback on the adventure. First of all, it mentions the use of italic text in the "how to use this" section but I didn't see any italic text in the room keys. Maybe an export problem or just something that was forgotten?
SPOILER BELOW
It was unclear to me what the NPCs wanted. Why are there goblins here? What do they want? What do the cultists think about them? And for the cultists. Do they know about H'thlux awakening? What are their plan? Do they know about the teenagers? If so what about it? About H'thlux, it says he is out to prove himself, how can that be done? I feel like these are all questions that could be easily answered by the module and would allow the Warden to portray them better.
And the premise of this adventure is that flies are plaguing the nearby village and coming from the barrow mound, but their is not any mention of where they are actually coming from? Some portal to the abyss? A magical nest producing infinite flies? This could add opportunity for choices. Okay we stop the flies but what about H'thlux?
Something fun to add could be a "what happens after" section. Like what happens if H'thlux is not dealt with? Or if the goblins escapes and start their own Orcus cult?
I want to reiterate that we had a lot of fun and I'm commenting for the purpose of giving back to the author in a constructive manner. Thanks for the great module!