Most of them (in fact all but Steamworld Dig where I got the idea from in the first place) do have the same publisher "Artifex Mundi" and are HOG. As far as I can tell they all use the same game engine working in a similar way.

Ah, yeah, that makes a lot of sense, then. I looked at the developers, but I hadn't looked to see if the publishers were similar. I'd say "all of them except for one are the same" counts as similar. :P

Another HOG bites the d ... without Steam client runnable - but this time it's not Artifex Mundi and it does not require a text file:

Brink of Consciousness Dorian Gray Syndrome (Collector's Edition)

Tested on win7/64

So what you're saying is that it DOES work without Steam? Because to my experience and to those of others it's NOT supposed to do that. Did you start the game once through Steam before trying it without? Maybe the DLC gets activated on the first run and then works even without the client...

Question: The first Two Worlds game used to contain Reality Pump's DRM, but this was removed one month ago. Can anybody who has the game confirm whether it can launch without the Steam client?

No, actually the first time I launched it was from the executable. I didn't even have Steam open, let alone logged in. The DLC was there (the arachnid portrait in my case).

That's strange. Until now I thought DLC doesn't work without Steam. Maybe they changed it recently and I haven't checked these games for quite some time or Stellaris is the only exception. Anyway, thanks for clearing that up!

In this case the DLC was a simple portrait. Steam automatically downloaded it with the main game for me, then the Stellaris launcher activated it. Really, there is no way for Steam to not allow it to work. If the launcher doesn't use Steamworks DRM then all is good. The only way I can see this not working is if Steam didn't download the DLC in the first place, which it fucks up on about 40% of the time in my experience, so maybe that's what happened to you.

Mountain does run wirhout steam client on windows as well.

Adventurezator: When Pigs fly does not need Steam to run.

Tested both on win7/64

Okay, we're all updated again. I tested and added Mighty No. 9 to the list, too.

Sakura Beach confirmed DRM-free on Windows.

STEPS DONE:
1. Set up a virtual machine (Windows XP SP3) with Steam on it and set it up so that the system is installed to a virtual hard disk, and the system can access the physical hard disk. Steam installed on the virtual hard disk (and thus, the virtual OS).
2. Download the game using the VM and to the physical hard disk.
3. Run the game from the host OS (Windows 10) which has no traces of Steam

After following these steps, the game is confirmed to work without Steam.

Credits to Seraphna of GalaxyTrail for the tip-off.

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EDIT: With the same steps above, Not the Robots for Windows is also DRM-free.

Now it's getting interesting because the following works:

Press X to not die (create steam_appid.txt and enter 402330)

You can play the game without using steam then and this time it's not a HOG it's not Artifex Mundi and it's not even the same engine. This game is more like a trash interactive movie.

Tested on win7/64

This is intresting: i know these HOG and some only work with client, lucky for me i got these at a sale.
Its a movie? too bad i am looking for more games that run once installed, i have a lot but i need more :D
Steam seems to have his summer picnic sale ?

I guess i be checking for cheap hogs, i checked the upcoming tab on gog but sadly its all 3d or fpp...
i guess the secret files and LH 1+2 will never come, so i be spending my leftover psc on HOG games.

No, it is not a movie it is more like an interactive movie with simple QTE events in it. It's nothing great but it's fun. I just stumbled over this game because of its ridiculous title (and the game also is pure trash).

Since I found out this steam_appid.txt trick I try it with every game I have by searching for the app id and then inserting this number into the textfile. I was really surprised that it worked with this one.

On a sidenote: imo Secret files is much overrated - at least I was expecting much more when I played it at the time. Lost Horizon 1 I liked - the second so far is much too pricey for me and the reviews weren't that good either.