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Dead Cells has a Daily Challenge or Daily Run mode, but it is just a minor optional part. Except that is required in order to unlock three weapons (“blueprints”) for the main game. Just by starting a Daily Run on different days unlocks the following blueprints:

1. Swift Sword (internal name SpeedBlade) the first time
2. Lacerating Aura (internal name DamageAura) after the fifth time
3. Meat Skewer (internal name DashSword) after ten starts

However, specifically the Linux version on GOG has an issue where it is impossible to start the Daily Challenge. The Linux version on Steam is not affected. To use anything online-related in Dead Cells – which thankfully is not a lot as it is a local single-player game – the GOG GALAXY client is required. Which does not exist for Linux. Until a GOG GALAXY client for Linux is released, I probably should not buy games from GOG any­more and just go with Steam, because otherwise I cannot trust that all features will work. Which is a pitty because I really like GOG for being DRM-free.

The Dead Cells issue is known for years and by now it seems rather unlikely it will be fixed. At some point a note was added to the store page:

Additional Notes: Galaxy client is required to access Daily Runs and Streaming features.

Let's hack the game

At the point of writing I was using Ubuntu 20.04.2 and the Dead Cells game version 1.14.0 / v24. You will need a hex editor which can handle big files of around 1 GB. I used and can recommend GHex. You can install it with:

$ sudo apt install ghex

Dead Cells is a game, it is really good, and it is available for Linux. However right after installation (version 1.0 from GOG) it did not recognize my PS4 controller. It could not be a problem with the controller itself or Linux in general, because the DS4 worked with other applications – for example it showed up perfectly fine in jstest-gtk (0.1.0).

After some research I came across this reddit post. Dead Cells uses the SDL library, so maybe that's it. I followed the instructions and built and ran sdl2-jstest. The output should contain an entry like this for the DS4 (2nd gen):

Joystick Name:     'Sony Interactive Entertainment Wireless Controller'
Joystick GUID:     030000004c050000cc09000011810000
Joystick Number:    0
Number of Axes:     6
Number of Buttons: 13
Number of Hats:     1
Number of Balls:    0
GameControllerConfig:
  Name:    'PS4 Controller'
  Mapping: '030000004c050000cc09000011810000,PS4 Controller,a:b0,b:b1,back:b8,dpdown:h0.4,dpleft:h0.8,dpright:h0.2,dpup:h0.1,guide:b10,leftshoulder:b4,leftstick:b11,lefttrigger:a2,leftx:a0,lefty:a1,rightshoulder:b5,rightstick:b12,righttrigger:a5,rightx:a3,righty:a4,start:b9,x:b3,y:b2,platform:Linux,'

… or like this for the DS4 (1st gen):

Joystick Name:     'Sony Computer Entertainment Wireless Controller'
Joystick GUID:     030000004c050000c405000011810000
Joystick Number:    0
Number of Axes:     6
Number of Buttons: 13
Number of Hats:     1
Number of Balls:    0
GameControllerConfig:
  Name:    'PS4 Controller'
  Mapping: '030000004c050000c405000011810000,PS4 Controller,a:b0,b:b1,back:b8,dpdown:h0.4,dpleft:h0.8,dpright:h0.2,dpup:h0.1,guide:b10,leftshoulder:b4,leftstick:b11,lefttrigger:a2,leftx:a0,lefty:a1,rightshoulder:b5,rightstick:b12,righttrigger:a5,rightx:a3,righty:a4,start:b9,x:b3,y:b2,platform:Linux,'

Take the value behind Mapping and add a line in your /etc/environment file like this:

SDL_GAMECONTROLLERCONFIG='030000004c050000cc09000011810000,PS4 Controller,a:b0,b:b1,back:b8,dpdown:h0.4,dpleft:h0.8,dpright:h0.2,dpup:h0.1,guide:b10,leftshoulder:b4,leftstick:b11,lefttrigger:a2,leftx:a0,lefty:a1,rightshoulder:b5,rightstick:b12,righttrigger:a5,rightx:a3,righty:a4,start:b9,x:b3,y:b2,platform:Linux,'

After the next reboot everything should be working. Or if you want to test it right away without reboot, then you can just add it to the start script of the game. Assuming you used the standard installation path from the GOG installer, the file is located at ~/GOG Games/Dead Cells/start.sh. Change the file so it now begins with:

#!/bin/bash
# GOG.com (www.gog.com)
# Game

export SDL_GAMECONTROLLERCONFIG='030000004c050000cc09000011810000,PS4 Controller,a:b0,b:b1,back:b8,dpdown:h0.4,dpleft:h0.8,dpright:h0.2,dpup:h0.1,guide:b10,leftshoulder:b4,leftstick:b11,lefttrigger:a2,leftx:a0,lefty:a1,rightshoulder:b5,rightstick:b12,righttrigger:a5,rightx:a3,righty:a4,start:b9,x:b3,y:b2,platform:Linux,'

That's what worked for me. If it still doesn't for you, try adding some udev rules as described in my article Using NW.js to communicate with a DS4 controller.