This is the best I've seen in regards to any kind of Thief-like gameplay. You get a gigantic city district with huge verticality (we're talking 7+ layers of substancial range), entrances on every level to I'd say 20+ locations ("friendly" and "hostile"), friendlies reacting to whether your armed or not, clever parcour sections everywhere, deep lore, deep story, latest visual effects like volumetric fog and even metroidvania-inspired items that let you retrace the whole mission for something new to find (that being optional).

You get your city mission, you get more traditional content sprinkled everywhere and you get your spooky fix every now and again but also in high dosage with a whole other district in something that feels like and "Undercity" but also achieves to completely make sense layout-wise in the end. In addition to the content it also looks stunningly beautiful with lots of detail and a unique aesthetic.

There is very little to critique about this mission: Some location entrances could be a bit easier to make out but that could be just me. I also thought the reaction of friendlies to the player being armed or not was not communicated at all beforehand, meaning that, if you're not used to missions differentiating between that, you just wouldn't know you could just enter a shop without being attacked on sight by the shopkeeper if you just sheathed your club beforehand. Some of the enemies in the lategame are also oddly silent in comparison to the city guards for example. You could argue that's how the author intended in regards to the reason these enemies are roaming these last parts of the game, but it seems more like an oversight, especially since it's not all of them keeping to themselves that much.
To be honest, that's just nitpicking on a most pedantic level.

If you're into any kind of Thief game, I greatly recommend you to try this as one of your first FMs. It's a bit experimental in how it delivers it's level design and gameplay and that can throw you off a little, if you're not much into venturing outside the Thief norm, but it's just too good overall to give a pass. To be honest, I had a hard time going back to other FMs with how much better this one fares in comparison on any level. I only played 9 FMs until now (including some of the highly rated ones) and they were comparable in regards of atmosphere, gameplay and story but Iris takes what were their 10/10s and puts those down a whole peg or two.

Can hardly wait for another FM of this author.