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Tried using proton experimental, GE proton, as well as proton 9. All of them resulted in a black screen on startup but audio would go through. Switching to Proton 8 not only ran the game and did not black screen, but ran it extremely well.

Switch from the native linux version to proton 9. Something
Bild ruckelte extrem und war blieb irgendwann schwarz.
Commands die empfohlen wurden halfen nichts. Mit aktueller proton Version 9.? Ohne Probleme.

Perfectly worked out of the box in the Steam Deck. Both in Handled and Docked mode. I was able to finish the game with no hiccups
Final boss / scenes on Docked mode, experienced some low/resolution / uploaded artifacts or unfinished loaded textures. But no performance degradation issues

LD_PRELOAD="$LD_PRELOAD:/usr/\$LIB/libgamemodeauto.so.0" mangohud %command%
The problem with the launcher can be solved by unticking the box at the lower left corner of the launcher that says: "Always show this dialog."

To get the Linux native version running: Under "Compatibility" settings, checkmark "Force the use of...." and select "Legacy runtime 1.0" Steam default to scout that is not compatiable with the last version from Ferial.

Native version refuses to launch. Proton version black screens
The native version running vulkan refuses to even launch to the Feral Launcher. The proton version (tried experimental, 9 and 7) just runs a black screen unless I put in WINED3D override in the command. Then it is just runs terribly. Shame really as I loved this game when it first came out.

PROTON_USE_WINED3D=1 %command%
When you try to look at these add-ons on the map in the game from the game menu, the images disappear, and a few times I had the problem of the character's hair disappearing in the main menu, but it was not a problem I had very often.
The game does not open without additional tinkering, but after entering this command “PROTON_USE_WINED3D=1 %command%” the game opens without any problems. I haven't tried all proton versions but I got better performance with proton7.0-6 than with other higher versions.

gamemoderun mangohud %command%
I have to switch to a QWERTY Keyboard
FPS drops to 30 or 40 depending on maps, probably due to the game being for console
If you use Proton 7 or Proton GE, this doesn't happen IIRC.
When using Proton 9 it starts a pre-game menu thing to set some settings. You can click "Play", "Quit" and change some settings. If you click play it doesn't launch the game. You have to tick the box "Don't show this again". Next lauch it will skip the menu and start properly.
Proton 9 works, just have to click on "Don't show this again on the pre-game menu"

Perfect experience OOTB
Game launcher scream 'Obsolete/Not supported Linux version' and 'CPU governor is not optimized for this game'. Just ignored them and continue anyway. As far as I can tell, the game runs flawlessly. Pretty good experience.
Decent bit of fun
Running great

mangohud taskset -a -c 0-3 gamemoderun %command%
At least with my hardware, some tinkering is needed to avoid horrible FPS drops.
The important bit I think is using taskset
to limit the CPU cores the game uses, this fixed the issue of the game running at 25-60 FPS and is now around 100 instead (the cap).
For my 5900X, taskset -a -c 0-3
seems to achieve a good result.

didnt properly detect screen as 2560x1440 but as 2048x1152

Game Properties > Beta > linux-last-gl-build
You have to use the beta linux build, straight boot doesnt open at all, forced proton doesnt load the menu
the launcher starts but not the game
i have installed "native" but when i hit the play button nothing happens... also im tried tu use proton but the screens goes black and nothing happens

PROTON_USE_WINED3D=1 %command%
The game launches to a black screen without PROTON_USE_WINED3D=1
PROTON_USE_WINED3D=1 will fix the black screen issue but it also causes an enormous drop in performance since it's forcing Proton to translate DirectX calls into OpenGL rather than Vulkan. In game benchmark results showed that the Average FPS on my PC was 3x greater just by running the native build so you should use that one instead.

PROTON_USE_WINED3D=1 %command%
The game will not launch in proton without PROTON_USE_WINED3D=1 which leads ot an enormous drop in performance.
Use the Native linux build instead. Average FPS tripled according to benchmark results.

I use Proton 9.0-3 and it just worked out of the box.

Works flawlessly out of the box
Do not tinker with it. When Steam installs a well known Windows game on Linux, it usually has a script that selects the preferred version of Proton. You can override it in game properties, sure, but in this case - just don't. If you do, and it's too new, there will be issues. Steam comes with Proton builds as old as 3.7, and that's for a reason. Don't be blindly switching to a newer one and then complaining something doesn't work.
Worked out of the box with Steam using Proton 9.0-2

Does not work on newer versions of proton (black screen on startup but can hear audio)
Had to used an older version of proton to fix the no video issue when using the higher versions.

40fps locked

Not entirely sure what dictates the failure to start sometimes, but the experience was good enough for me to finish the game.
When lightning strikes, all water puddles and similarly moist surfaces change into a rainbow-y oil texture.
Sometimes it takes multiple attempts to start the game.
Also, most of the times when quitting the game keeps running after closing, forcing me to stop it from steam.
Some information about my setup: I am running Gnome 46, on an Nvidia laptop, with the entire xorg session being rendered using the Nvidia card. I also have my laptop hooked up to a second external monitor. Played through the game with a ps4 controller.

Got it to run silently with 6W TDP limit using low settings and 80% res. Still looks good and plays great with 4+ hours of battery life
Disable FXAA, doesn't appear to work properly and has a much larger performance penalty that it should, about 10-15% or so decrease in fps
no problems here, runs out of the box

Black screen
I think it's playing intro and I was able to cut scene, not sure, all black.
If you searched steam community of the game for "black screen" you will see numerous reports, even in Windows.

Inicia y funciona perfecto: Completado al 100%,1080p,graficos(alta),control(xbox one/series),Proton=8.0-4
IMPORTANTE:
- Activar y jugar la version de windows.
- Aplica para cualquier juego nativo de linux.

LD_PRELOAD="" DXVK_ASYNC=1 gamemoderun gamescope -w 2560 -h 1440 -W 3440 -H 1440 -f -- %command%
I like to tinker but for this one probably don't need to. Added gamescope for better control over game resolution.

Game sometimes switched to keyboard input after cut scenes if no button was pressed, pressing any button on Steam Deck during the cut scene would prevent that. After the switch to keyboard input some buttons would not work and I had to relaunch the game.

Worked perfectly fine, beat the game with no issues and no configuration was needed.

Usually reloading fixed the audio issues- which would sometimes totally cut out.
Went completely through the game on Linux.. Very enjoyable and stable experience. I believe I could only note one crash during a cutscene near the end of the game.. Sometimes audio would cut out and would have to reload. Othewise, it ran incredibly well.

mangohud %command%
I am on KDE so I had to turn off compositing because I see diagonal screen tearing. Once you turn it off, looks just like it would on Windows.

protonfixes is needed because with proton 8.0 it doesn't install c++ library (microsoft redist). Installed via AUR and in the proton 8.0 folder you copy conf file and add import protonfixes at the bottom. For more information see the github for proton fixes.
It won't work without protonfixes on proton 8.0. You will get a c++ error.

A fairly exceptional Deck experience for an un-updated, nearly decade-old game. Minimal issues, and a perfect fit for on-the-go play
Certain flavor text can strain your eyes a bit with how small it can display
The game is quieter than you'd expect, even when maxed out. ~75% was my sweet spot with headphones and earbuds, but 100% was where I'd leave it without them
On a couple of occasions, all controller prompts were swapped out for keyboard and mouse ones; all but one instance occurred immediately after I put away my Bluetooth earbuds. The game needed to be restarted if I wanted them to show up again. The only other time this ever happened was after I booted directly into the game from sleep mode one session. Not consistently replicable, though