Interesting.

But where did the barrier come from? That needed seven humans to shatter. Or one human soul and one Monster soul to pass through?

In this case, the barrier is a real physical obstacle. The wall of the mountain itself.

I suspect it had to do with either entertainment or study on codes, possibly both, considering how vast his intellect was.

Hey. I just wanna say I really like this story. It's an interesting take on an Undertale AU where the monsters are human...or at least, WERE human.
The journal entrees gives a good idea of how long the transformation is taking place, as well as what's going on in the person's head as well as with their bodies as they become a monster. The names you came up are pretty creative for each human that becomes the monsters we know in game. It especially gives me an idea of who's who(and who becomes who). Although, one of the people I don't really recognize at all; Dr. Leeland Pierce. Is he any specific person that becomes a specific monster I don't recognize or is he just an "expert" dropping foreshadowing in the story?

I like how Mount Ebott has a different name and that it's "focal points of the ancient ley lines"; it really gives it a magical feeling about the mountain before even climbing/entering it. It especially lends to the idea of how it will transform any human that enters there. The fact that there seems to be physical evidence of monsters roaming on the surface of the mountain kind of set up a feeling that monsters may have been there a lot longer that what would be guessed. I would assume the golden flowers to be buttercups, but if the experts couldn't even tell what it was, could it be a new type a flower, maybe a magical one? Even on the mountain's surface, there's mysterious/unusual flora.

Even when exposed to the spores for 1 day, the effects are already starting, beginning with hypnotic/tranquilizing properties. I like the reaction recorded by Aileen towards the underground beyond where they had fallen and then comparing it to a fairy tale; kind of what Undertale's setting sort of is. I also like how, despite the fungi's sedation, some of the more aware are taking note of the odd effects it's having. I find it cool how some are transforming into monsters faster than others, how some adapt to magic as they gain power over it, and how some are loosing their human memories faster than others(except for Dr Masters, who remembers everything). The description of how each character writing the journal entry felt as their memories gets muddied and forgotten really paints a picture in my head of what's going on in theirs as this goes on. I also like the notable, yet slow shift in personalities as they transform. At the point where they stop keeping track of dates makes sense, since it seemed like at that point, they won't be leaving or at least not leaving as humans. It was cool seeing everyone's point of view of how they reacted to their own changes; from Asmodeus being disturbed by how painless his transformation is, to Terra's guilt over causing Billy's fire elemental form, to Dr Masters assuming his death his coming due to his skeletal transformation. It was also cool that you even was able put in the "Dark, darker, yet darker." quote in the story. It's sweet that some of the monsters left behind still care for the others, though I'm not sure if this has something to do with the fungi's tranquilizer or with their transformation.

I see that the River Person is just as mysterious as ever, as if they have always been there before the expedition team fell down and when there might have been "more activity" in the mountains in the past. The "Boatman" name gives me an image of the River of Styx, which makes sense given the context where Dr. Masters is meeting them; especially as they show Dr Masters things I doubt I could imagine. The scene where Dr Masters encounters a Gaster Blaster(fitting name for what happens) feels like it was both exposing him things(like his own skeletal hands or new knowledge of the underground) and magic he has not seen as well as feeling like Dr Masters is being gifted a Gaster Blaster. It feels like now Dr Masters is taking up a special role in the underground. Kind of sad that as much as Asgore tried to at least hold on to his old name, even he forgotten it in the end, leaving Gaster as the only monster who remembers anything of being human.

My favorite character throughout the story is Gabriel Masters. I also really like how competent Asmodeus Bangor is when it comes to leading the exploration team figuring out what to do.

Some things I want to point out that I find humorous. Though in the games Asgore is the one who comes up with uncreative names for places, I find it funny that it's Dyna that came up with a lot of the names for the monsters. Like Aileen said, the names are like their monster forms, "a mishmash of parts from our old lives and this … new change". I laughed when she was giving "Dog-" names. Also, no matter what form she's in, Undyne's forever a bad cook. I can't help but to imagine that the expedition’s insignia is quite similar, if not exactly, to the Delta Rune. The realization for both Terra's and Asmodeus's feelings for each other is amusing; as if they just discovered that they like each other and is surprised by the realization.

I'm sorry if this comment is coming off as a summary. I really love the story, how well written it is, how detailed it's written, and how overall well done it comes off. You are a great writer and thank you for giving us this good story.

Wow, I realize I never got to answer this properly. XD I feel like such an idiot for missing this. Sorry about that, Bitty. To answer your question about that one doctor, I believe he is the one that ultimately becomes our robot friend, Mettaton, though I could be wrong. It's been a while since I wrote it, so I can't recall for sure. It may just be a name I gave a random scientist.

Thanks so much for the high praise! :D

This is begging to be made into a Plague Inc. custom scenario if the monsters got out and started to spread the Spores, can I have your permission to potentially make one?

What's Plague Inc.?

Ah, nevermind. Just looked it up. As for using these spores as an idea for applying in a Plauge inc. setting, knock yourself out. It sounds like fun. I know someone else asked to use one of my spells I've written in a D&D setting of their own design. I don't see why this should be any different.