Brainchild completely breaks all FM conventions in favor of something abstract, off-putting, and plainly strange. To my eyes, that's a good thing.
It's interesting playing this in 2025, when megastructures have become commonplace in indie games (Lorn's Lure and V.A. Proxy being some examples of the trend) -- Brainchild was two decades ahead of the curve there. You spend the entire mission wandering through a seemingly infinite megastructure, with no context as to the literal or figurative meaning of its existence or the meaning of the mission itself. It feels like a trip into Hell, in some ways, with its use of pitch-black darkness, strange geometric shapes, hissing creatures in the distance, and cold metal at every turn. It all incites a unique sense of dread and discomfort.

I will say though, in many ways, it feels like a first draft. It plays as a walking simulator (hey, again, ahead of the curve for 2002), with some light key hunting and platforming. It's about as barren on the gameplay front as the environment it's set in. While that does add to the intrigue of the mission in a lot of ways, I would've loved to see the author expand on this concept, but alas, they haven't posted in more than 20 years!
Even so, I'd love to see the talented members of the FM community try their hands at weirder concepts like Brainchild's, missions that toss aside the mansions, manses, and manors of The City in an attempt to bring Thief's gameplay to new places and new creative heights.

I love the absolute mercilessness (and freedom of imaginative fantasy and mystery it gives you) of this in terms of absence of any context, you have to parse any interpretations of the place and where your avatar is coming from on your own at your own risk. To my knowledge there is nothing fully comparable among Thief FMs but slightly more "conventional" ones that have some aspects of this are Katharsis and some of DrK's missions (also there is a mission 3 in a campaign called Night Watch with similar tones).