An enjoyable short mission for about 30 minutes of gameplay.
It definitely sticks to the horror theme and there are quite a few interesting gimmicks.
The gameplay is basically "run from A to B and the next door will open". Sometimes it feels more like a movie than a Thief mission, but it plays surpsisingly well. There was one unnecessary switch hunt and one annoying quickload contest in the second part.
Most locations look rather crude as apparently more attention was paid to the trickery.

Despite all its flaws, the premise seems to generally work so I'm giving it a 6.

PS. I totally don't get the "hardest" tag. There's one problematic place, but other than that I wouldn't even say "challenging," let alone "hardest."