As D'ARKUN combines horror and mystery, which are two of my least preferred genres, I'll mention I still enjoyed it, because it gave me several good chances to. I'll tackle the programming side first. On realizing D'ARKUN was, in fact, in Dialog, I realized it was the first Dialog game I played. I had clear chances, since others have appeared in IFComp. I left my first run-through thinking "Wow! How did the author do that?" to some parts I found unusually smooth. Now the programming side is well more than competent. It certainly gave me ideas of stuff to do in Inform. And I think Dialog uses very well the information of what programmers need and use from the Z-machine, as well as more data on what players find improves their own experience. So Dialog and such aren't bound to support arcane ways of doing things just because Infocom did it that way, when maybe Infocom only did it that way due to hardware limitations. Hooray, progress!
But there's neat stuff which the author seems to deserve credit for. The big boost I saw in D'ARKUN was the "find" command, which helps make a big in-game world such as D'ARKUN feel much more accessible. FIND X moves you to X's location, if you can make it there. This is something I implemented as a debug command in some games, but it was tricky, and it felt smooth here. It even rejected my attempts when I dropped climbing gear needed to bridge gaps or travel between towns. This all set the table for a much more pleasant experience than I feared, but it would have been good in Inform as well.
D'ARKUN takes place in a small set of villages near the north tip of what was formerly East Germany–a great spot for an obscure, distant cult to take hold and go about their business for years without anyone noticing. You generally ride your bike between them – I'd have liked maybe a menu or shortcuts here so I didn't have to type "ride to altenkirchen," but I did enjoy not having to do this too much. Though I was maybe sort of hoping for nudges to say, okay, you spent enough time here.
After a good competent start on my part, I went to the walkthrough very early on this one. Enough was signposted in the game and not the walkthrough that I enjoyed reading the auxiliary materials that popped up to give atmosphere. They provided stronger atmosphere than some passive verb construction during action sequences ("some hands are grabbing you") – which looks like a translation thing that's easily fixed. And I think sometimes it was hard to follow the why's of the walkthrough. I had to search instead of look at a lot of things. HINT mentions this, but still, it was a bit of a nuisance to me and one of the relics of ancient text adventures that is on the author and not Dialog.
The puzzles that appeared were not super-esoteric. A lot revolved around using the climbing gear judiciously. But stuff like mixing the right liquid for the final bit felt like trial and error. Still, once I dropped down into the final tomb-ish area that there seemed no way back from, it was appropriately creepy, and the escape was believable. The bad guy was, indeed, bad (a variant on "What you think is evil is actually power you're just too scared to use" that always seems to be effective) and information along the way built up to who he was and what he was trying to do. Diaries scattered around also gave me an idea of past events, and perhaps the most interesting part for me was a chair you could sit in for a psycholgogical evaluation, which was simultaneously creepy and useful.
I'm at a loss to say too much about mystery/horror games, as I don't really grok their conventions and so forth. Other reviewers note D'ARKUN is even more in the Anchorhead vein than I'd guessed, while still being its own game. I can't say, because I haven't played Anchorhead--in fact, Cragne Manor with a walkthrough was enough for me! But D'ARKUN plus a walkthrough (even one that doesn't get all the points) worked as a positive experience for me, as an outsider. The password-protected PDF, of a map you unlock on your second day (D'ARKUN uses sleeping after performing tasks as a way to provide chapter breaks of a sort,) is a neat way to make sure people don't spoil too much too quickly. I did find the light-requiring puzzles tricky given the time you could keep the lamp lit. I wound up save-and-restoring, even with the refill I found later. But they weren't too bad, and I was able to accept not seeing a lot of the game beyond the walkthrough that got you half the points. I had some idea of places I hadn't explored, and the ending was satisfying enough.