This will probably be a very long review, because there's a lot to get your head around in The Perdurance and I do feel a degree of duty to do its very weird collection of pros and cons justice. If you want a tl;dr: Is it good? Yes. Pretty damn good, actually, but its few design flaws work against it massively, and it could still be better.

[NOTE: By necessity, this review includes spoilers for the plot in The Perdurance. Additionally a big part of this review is predicated on the assumption (made with about 85% certainty) that what I will later on describe as "Act Three" of the Perdurance was a scripted event that was intended by Skejven. It may be just possible that this is not the case, and that Skejven or someone else will correct me on this. If they do, I will update the text of the review to reflect that, but I will probably not change the score given because I think Act Three being intentional is pretty equal on upsides and downsides for the FM as a whole.]

This is an odd FM because it was ostensibly created by one person, and yet there are two very distinct design voices throughout it that give the impression of two designers of vastly different skill levels and attitudes. For a bit of levity, and the sake of articulating the pros and cons of the FM, I will call these two personae "Pinky and The Brain".

The Brain is a genius, or something close to it. As a modder he has a seasoned professional game developer's understanding of level design, pacing, atmosphere, implicit storytelling, player psychology and how to integrate effective horror into Thief 2 without overwhelming the experience. Every one of his additions, once identified, is a pleasure to experience and thought-provoking well after the fact.

Pinky puts things in the FM because he's seen them in classic horror games and that's what horror games have. He doesn't care if they fit the premise of the FM, Thief 2 in general, or Brain's contributions.

PLAYING THE MISSION
The Perdurance, at least in my experience, follows a very noticeable and pleasant three-act structure. In Act One, Garrett boards the ship from his little rowing boat, finds the front doors to the main lounge barred and must effect an alternate entrance through the bridge. This is where he discovers his first dead body, and the fact that something has gone very very wrong. From there, Garrett sweeps the staterooms and corridors on the top two floors of the ship to try and meet the bulk of his loot quota, finding more disturbing scenes of death (including a particularly gruesome suicide-by-hanging that was unpleasant to look at but not *completely* gratuitous in its inclusion given what we will later learn about what's happened here), valuables and readables. Much of the geography of the upper two decks is illustrated with blocked routes and obstacles created from smashed furniture and collapsed or blocked doors.

We also discover where the Mask of Nerlatop is- it's in a safe, one that needs a mechanist gear key to open. That will have to be found, and the last of the loot quota made up, so down to the lower decks Garrett must venture. But first I'm going to talk about...

LIGHT
The Thief series has always had a bit of a complex relationship with diegetic light. Darkness is your friend, after all, it keeps you safe and hidden. But like most beings you need light to see, and you're looking for things you need to see. Additionally, as soon as a foe has detected you and makes to attack you, darkness can become your enemy because it hides your attacker as well as you. You will be keenly aware of this here.

Simply put: The lighting in The Perdurance isn't very good. Most of the map is WAY too dark, there are few to no means of changing this in game without adjusting the gamma (definitely none in the upper decks) and the lights that there are often flicker constantly at high frequency, because (*Pinky voice*) flickering lights are scary and they're what horror games have! I am not epileptic or medically photosensitive (to my knowledge, though as we saw late last year with a certain big budget game release that kind of thing can be latent and unexpected) and neither was anyone else watching, but we all agreed the strobing lights were hard to look at for more than a few moments and more annoying than actually scary, especially since they meant that scouring the locations they were present in for missed loot would be necessary several times. It has a reasonable diegetic explanation (the electrics have been damaged in the scuffle with what lies below and the ship is probably running out of power given how long it's been drifting) but it feels largely unjustified in terms of level design, at least in act one, and as Pinky's primary contribution it feels out of place among Brain's thoughtful and modern efforts at route creation, implicit storytelling and environment layout because lighting of that sort is something that has largely been abandoned in professional game releases since around 2005, for good reason.

ACT TWO
Garrett ventures downstairs to the lower decks, home of the crew berths, kitchen and engine rooms in the hull. Here he discovers the source of all the death and destruction: Undead monsters!

I've made no secret in the past of my distaste for the undead in Thief. Coming to the series as an adult I have no particular nostalgic fondness for being frightened as a nipper by the Bonehoard or Return To The Cathedral, and most of the reasons I find evading capture feels good and triumphant in Thief simply do not apply to evading the undead or non-human-aligned monsters. I am delighted to report, therefore, that the undead in The Perdurance are, mostly, REALLY GOOD. A custom model, seemingly (or at least a non-vanilla one) the monsters down here are seven-foot tall humanoid masses of skinned minced flesh with skulls for faces that behave like less intelligent Haunts. Probably. It's difficult to see them given how incredibly dark it is everywhere they go. These monsters feel scary as hell: they constantly patrol, they're horrifying to perceive and they're incredibly powerful combatants (they might even be invulnerable to attack- I've had problems with Thief 2 zombies in FMs- including FMs by Skejven, for that matter- not registering sword hits when they should in the past so I'm not prepared to say this for definite, but it doesn't matter anyway, because fighting them here isn't really a viable option; they're incredibly beefy, they can attack faster and harder than you can and given the environment sneaking up to backstab them is almost impossible) so you have to watch for their circular patrol routes and listen for their sounds to learn where and when it's safe to move.

And here, unfortunately, Pinky sticks his oar in again. The ambient noise that the monsters make is the staticky babble of an untuned analogue radio. Why? Because that's what horror games do! Who cares if it's a jarring inclusion that makes no sense? Things that shouldn't make the noise of an untuned radio making the noise of an untuned radio is scary, even in environments where radio technology is as of yet too limited to have created the context for that noise!

Still, it's a distinctive sound, and one that you'll need to herald their arrival because below decks is even darker than the corridors in act one. The monsters move on circular patrols, so the best way to avoid them is to follow one until you reach somewhere you can peel off to an area they do not go. There's some loot here, as well as two different means of access for the hold, where the engine (and our prize, the gear key) lie. More monsters lie below, amongst a simple collection of props that tell an easily understandable story about where they came from and roughly what has happened to this doomed vessel. Poor fools!

The key we need is by the dead body of its carrier, and takes some tense timing to grab as monsters patrol in circles both on the hold floor and on the gangways above the engine. Thankfully this area has many places where you can effectively stay hidden until you learn the routes and frequences of the ones you need avoid. Grab the key, grab some extra loot to make up the quota, and escape back up to the safety of the upper levels! Yes!

ACT THREE
Safety? Haha, no chance. There's one of them up here now, patrolling around outside the cabin with the safe in it. You'll have to get past him before you can pop the key in and grab the mask, and then get out again past him.

In principle, this is a REALLY GOOD piece of design. In Act 1, we encountered the geography of the upper decks and were made to examine it closely. In Act 2, we were shown the monsters and allowed to observe their behaviour. Act 3 combines the two, putting a monster into the previously safe environment we're familiar with and changing our relationship to it. Spectacular.

In practice, this is unfortunately a little more frustrating than spectacular, for two main reasons: First, we've been taught up to this point that the way to evade the monsters is to study their patterns of movement and action and work around them. Unfortunately, up here in the upper decks it's too dark to do that well, and the non-circular nature of those decks means that this guy doesn't patrol round and round like the others, but rather back and forth, which you can't see or judge from where you enter the area. Secondly: It's very difficult to change his route by attracting attention or even to approach the cabin door without him discovering you by bumping into you because the corridors on the upper decks are too small, tight and linear. There's nowhere to hide until he passes that isn't either directly in his way, or so close to somewhere he'll stop or the only constantly lit light that he'll see you.

Getting past this monster was an exercise in frustration and eventually I found I had to cheese it a bit by hiding somewhere he couldn't path to in the cabin until he got bored and went away. But once I opened the safe and grabbed the mask, then it became a mad dash for the exit! He was right behind me all the time and I had only the vaguest sense of where to go, but finally I ran out onto the bridge again, hopped through the broken window he could not follow through and laughed as I turned to see him standing forlornly looking at me from the site of the captain's body, actually flipping the bird to my screen.

So it is, in the immortal words of @dril, "impossible to say if it's good or not."

So, what do I think? I think The Perdurance is a good, tight, well paced and plotted piece of design that a lot of thought has been put into. I think The Brain is the real Skejven: an expert crafter with a good eye for gameplay, atmosphere and flow. (Which is, admittedly, quite an incredibly funny thing for me to say given my views the last time I reviewed one of his FMs, but I mean it!)

So what's my explanation for Pinky? Perhaps this veers too much into armchair diagnosis, but the Pinky components feel like a confidence problem, like Skejven was worried that the FM wouldn't make a compelling horror scenario or game experience without some hallmark tropes from established horror games. I think this is wrong, and those parts actually harm the FM and its atmosphere more than help it. It's plenty scary and thrilling enough on its own without them! I'm interested in seeing more from Skejven, truth be told, because if indeed these are confidence issues then they can surely only improve, and future FMs will be all the better for them.

For this FM specifically, my advice would be to light up the upper decks more, cut flashing lights entirely, find a more suitable noise for the monsters to make, and perhaps to add a few more through routes up top so the final monster can be evaded or distracted via player planning a bit more intuitively. The lower decks can probably stay as dark as they are, though maybe more could be done with the low red light found in places.

All in all, The Perdurance is a solidly good FM, but its flaws are noticeable and debilitating, which is a damn shame.

Thanks for the review
About the flickering lights: I didn't really follow any "standard" horror tropes with that one, my main horror inspirations were Silent Hill and Lost in Vivo and tbh I don't really remember much flickering light from those, I just thought they do provide nice variety in lighting plus they also indicate that a lot of the the lights are malfunctioning, like a lot of different stuff aboard the ship.
About the sound monster makes: aboslutely can understand where you're coming from, but I think it sounds cool, gives them more unique feel and because this is a horror mission, I do have a little bit of freedom to use more abstract audio/visual themes
The monster at the top level I think shouldn't be that much of a problem because you can use a vent to get to the bedroom. One thing I would change is probably to not make the vents be of metal material, while it does make sense (and that's why I made them that way) I think it kinda hurts the gameplay cause they're supposed to reward you with a safe way of travelling between rooms, but because they're made of metal it can be easy to alert the monsters anyway. Probably annoying for ghosting folks

You CAN use a vent to get into the bedroom... if you know it's there. But this is unlikely to happen because both ends of it are just too dark. I just went back and looked and evening knowing the room had a vent in it I still had to guess and scrub along the wall to find it. And I normally play with my gamma above default! Goodness knows how other people will find it. Kinda feels like it should be visible on one end- I'd say the bedroom end, so you're rewarded for remembering it when it becomes relevant.

YMMV, I suppose, but I've never felt like darkness is a particularly fair or challenging obstacle to put in a Thief mission (and this applies to Looking Glass as well who bear the brunt of the blame here) because the player lacks any meaningful way of ameliorating the obstacle or operating within it, even at cost, that isn't just "turn up your gamma and cheat your way out". LG made a halfhearted attempt to address the problem with flares, which aren't available everywhere, and of course some missions have light systems that can be turned on and off, but the fact remains that there is a line between "Wahey there's lots of darkness in this mission so I can hide easily to act from the shadows" and "Everything's too dark and I can't do anything about that, so I can't interact with my environment because I can't see it."

Always struck me as a little strange that LG never equipped Garrett with a lantern, to be honest. It has the easiest and most natural balance cost of any tool he'd ever have had.

Oh well. FM's good, on balance. You should be proud.