Heyo, figured i'll post this as a discussion thread so its easier to keep track of than losing it in the comments. Here's a playthrough of this map done on Nightmare difficulty.
(Vanilla except for All-Talk being enabled)

Also comes with a bug report: First player respawn point at the White Pillar room as you enter the mantle area doesn't seem to actually work. Displays searching for spawn point but its unable to find a spawnpoint. You can see the exact spot around 7:55 in the video.

https://www.youtube.com/watch?v=7mKbdwsp16Q

For Feedback, i thought the map visually was quite striking, especially for a first NMRIH map. Judging by some of the tricks used I assume this isn't your first source map or experience with hammer. It was fun to playthrough and explore because of this, and The Core's skybox area is particularly striking. It even managed to invoke a weird sense of creepiness/dread to it which was nice.

The gameplay suffers a bit however, as a lot of the areas tend to be super wide and easy to get around zombies. While not game ruining it would have been nice if there was more in-between objectives as well instead of just "Continue down path", which would have added some spice to the bits. I love that there is a kill trigger for the zombies near the ledges allowing you to push them off though, so much more fun than the usual "Stagger at edge but don't move" that normally occurs.

Final Area's interactable terminal could maybe use a bit more clarity. I love the Rain World Easteregg in there(i mean i guess the whole map is one due to the inspiration), but the terminal wasn't 100% clear when it was actually finished being interacted with. Having a more clear indication you just need to wait would solve some confusion, but its not a major issue.