A Sinister Night is an attempt at replicating exploration horror gameplay in Thief 2. It removes all of Garrett's weapons and items, save for a "lantern", which I was delighted to see after playing other FMs that have used the concept to help surmount the problem of darkness hindering the player as much as helping them.
Tragically, this lantern functions a lot more like the flashlight of your typical atmospheric horror game, illuminating whatever Garrett is looking at... sort of. It seems even the NewDark engine is ill-suited to this kind of mechanic, being unable to display the light naturally and illuminating specific objects more frequently than actually casting a circle of light. This means that the light can flicker between the foreground, background, walls and floor as you move it around, which is rather nauseating and difficult to follow. And it's not just a poor implementation of the gimmick! Some objects in the FM have interactions that only occur when they are illuminated, so the directional nature of the lantern was absolutely built around. It just isn't very good or usable.
The actual gameplay, such as it is, involves navigating inside a stereotypical spooky mansion while the FM plays unsettling music and creepy sounds at you. It's all a bit over-egged and at one point a distant jumpscare involves a straight up jpeg of the Grunt from Amnesia, which did not give me incredible confidence as to the maturity or skill of the FM's creator.
Already annoyed and hamstrung by these technical difficulties and poor-taste decorations, I eventually decided I had had enough when the spooky music touched on my nerves, but don't be fooled: This FM scared me away because I am a massive baby who can't handle even the most hamfisted horror. Horror fans or even enjoyers of competent Thief 2 FMs will find nothing attractive here.