A demo that's definitely worth a go - the unforgiving cave-exploration survival simulator Casualties: Unknown has a really interesting idea.
Casualties: Unknown sets up an interesting world - one where humanity has scientifically progressed so far that we created an "Experiment species". And of course, plenty of unethical things have happened. You're part of this. On a distant planet some special experimental super tech fell down into some caves, and as a testing specimen you're being sent down to try and retrieve it.
What makes it interesting is the injury system. You have a simulated Limb and Cardiovascular system, with individual injuries and infections possible at any moment. That includes the psychological side of things too, having a bad time could just make it even more challenging.
Check out the trailer:

Direct Link
The UI could definitely use some work, but the idea of it and the way you interact with everything and how you treat injuries makes it firmly worthy of blasting through the demo.
Game Highlights:
- An entirely simulated cardiovascular and limb damage system.
- Character traits to increase on your run: Intelligence, Strength, and Resilience.
- Robust crafting mechanics to increase your odds of survival.
- 3 characters with their own distinct playstyles.
- 11 layers of an alien planet with their own challenges and dangers.
- 300 items to find on your runs.
- Adjustable loot and danger values accessible on the main menu.
You can try out the demo on Steam now, which worked on Linux with Proton 11.




Anticheat check - which competitive games actually work on Linux?
How to give Valve feedback when Proton games have issues on Linux / SteamOS