After escaping from your cell, you delve downwards. There's plenty of filth and gloom, but that's far from the worst thing you could find down there.

This mission has great horror. There is a particular method I've noticed in at least one other of the author's missions. It really works, especially when coupled with strong visuals and sound design.
There's also a powerful sense of the unknown. I don't understand the nature of some of the things I encountered in The Unseen, only that they must have come from some terrible maw deep under the earth.

On a personal note, anything that involves going ceaselessly down, into places more terrifying and unnatural than the ones that came before, will resonate with me. It puts me in mind of games like Rogue, NetHack, Eye of the Beholder. There's just something I love about this form of progression, where you're steadily losing your grip on the world above.