I discovered that Abyss: The Wraiths of Eden can be converted to DRM-Free by adding a 'steam_appid.txt' file in the game directory and writing the number '284710' in it. It's the same solution as Nightmares from the Deep, so I guess all games from Artifex Mundi work like this. If anyone has more games from them, I'd would be nice if you check adding the file with its app_id on it and check if that works.
It seems that the sequel of Nightmares from the Deep works with this trick too. It only needs to have a 'steam_appid.txt' file in the game directory with the number '284790' in it. If I manage to buy more games from Artifex Mundi I'll keep trying this method to get the games working DRM-Free :)
Edit: Whoops, the game I'm talking about is Nightmares from the Deep: The Siren's Call
. The sequel, as I mentioned before, of Nightmares from the Deep.