A simple but effective two page mission, evocative in its setting and mission background. 

• For being only two pages, it's very economical with its layout choices. Everything is just a glance away, making it easy to see what is going on and what a ref could throw at the players next.
• It's taken a simple prompt and really expanded on it, giving it a new sense of history and background. It feels perfect to slot into the Cold War paranormal activity of FIST. This background information is great for a beginner ref to start to think about how they could expand the mission in their own way: What if the men who beheaded Anahid were part of an order of knights who are still lurking in the shadow of the mountain? How could FIST help Anahid to escape from her eternal torment at the bottom of the pit? 
• The mechanics of the pit itself are a bit too loose and contradictory to be run with ease. Are FIST traversing the ruins of buildings that have fallen into the pit already? Or are FIST just climbing down a wall forever, every so often dodging falling debris? When do FIST actually reach Anahid, because the mission doesn't seem to specify a time - what triggers the boss battle?

I think a few more pages and this mission could be great; expand the random tables and dangers, flesh out each location they might encounter a bit more to give the navigation a bit more meaning, and give Anahid a bit more background to make her fan two-phase boss fight have a bit more meaning.

For starters, this is a cool prompt, and you've done some cool stuff with it. I like the idea of the psychic tendrils, and the idea of including a table of local people and places that have been drawn into the void is a really natural fit to demonstrate the impact of this boss. It's also super sick to put the growth of the hole on such a short timer. Four hours until it consumes the entire world? Whoof, that's heavy. Classic.

I do really find myself wishing for more, though. It does feel a bit like a random collection of obstacles tossed into a hole like action figures—not necessarily a bad thing, and I'm sure for some GMs that's exactly what they want out of this kind of mission, so if that's what you were aiming for with this, then, hey, great job. This is definitely a mission that I could run and have a great time with. The Ottomon Skeletons seem like a fun encounter.

That said, I personally would have been more drawn into the mission if there was a richer connection between the people, places, and objects that have fallen into the hole. A stronger sense of community, maybe. More exploration into the psychological state of Anahid the Beheaded, and how the objects she draws into the hole relate to that. Perhaps a stronger characterization of the hole itself. Maybe each building could have been a little more fleshed out. These are all just some subjective ideas as to how I would have made this mission, though.

I do agree with Wyn's comment. It's a bit vague how players are meant to progress through the hole, beyond just "fall down." Is there a set number of encounters they're meant to have before running into Anahid? Is the GM meant to play it by ear? Not a huge problem, as I'm sure a competent GM wouldn't be running a mission blind out of the book and would have some amount of time to iron things out ahead of time, or at least could improvise, but some more guidance would be very helpful.

The layout is perfectly serviceable. Given the time-frame of the jam, I don't expect everybody to have made the most gorgeous layouts ever (I tried to do that and didn't finish my entry in time! Whoops! Still working on it). That said, it's a bit plain. Gets the job done, not a problem if you just want to get your stuff on a page and put it online. I haven't looked at your other work yet, so I don't know if this is moreso just a product of game jam crunch (apologies if it is), but if you would like to pursue more technical layout work in future projects, I had a lot of luck looking through NotPotableWater's Rolodex of Free Design Resources (which I assume I can share here, though I'll delete this link if asked to: https://docs.google.com/document/d/1JDbKTa2xBKVAZE7Dy5BNLjfbHocJhyMcGluOQAKU9KA/) which was sent in the FIST Discord's thread for the Gashapon Jam. In particular, I've been enjoying using coolors.co for color palette creation. That might be a useful tool to make your projects look a bit prettier, if that's something you want to invest your energy into. Pretty graphic design is definitely more of a nice-to-have than a requirement, though.

Hopefully this was helpful for you!

The overall concept is super cool, I love the idea of freefalling onto buildings and fighting through them, and the boss concept is great.

I'm still pretty hazy on how to complete the mission, though; if the pit is bottomless, how do the players reach the bottom to fight the boss?