So, the developers of this game (that nobody liked) decided that latecomers won't even have a chance to begin trying to enjoy their game because of how outright hostile they have made it to anyone that didn't pre-order it on Kickstarter multiple years before its release. Tons of DLC items are locked out of accessibility to anyone picking this game up today. Now, most of these items are just flavor trash or a different skin for a pet, but one of them is the Adventurer's Backpack. The item that doubles your inventory space and happens to be the ONLY item in the entire game that goes in the Backpack slot, that's right, unless you bought the game through Kickstarter for a minimum of $75 you won't be able to obtain this item.

This malicious business practice of locking out a crucial item actively encourages people to pirate the game because every pirated copy of the game has all of the DLC unlocked by default. So yeah, this game that nobody likes does everything it can to make sure people don't buy it, because if you decide to actually play it you're going to want the only item in the game that goes in the empty backpack slot on your character.

So if you're coming across this and you've had the misfortune of having paid for this game instead of stealing it, and you want the backpack but don't own a time machine to pay $75 for it years ago. What you can do is:

1. Go to the storage chest in the Midnight Forum, that's where the DLC items will appear if it's done correctly
   
2. Create a few backup saves in case something goes awry, you might want to empty the contents of your storage chest into your inventory before doing this, not sure how it handles adding new stuff to the pile.
   
3. Obtain a totally legal DRM-free copy of the video game from a friend that had the fortune of purchasing the game through Kickstarter and has all the DLC unlocked
   
4. Keep the two versions of the game you have separate (your steam copy in your steamapps/common folder, and your friend's copy somewhere else)
   
5. Go into your steam copy's folder and find the Underworld Ascendant/UA_Data/Plugins folder
   
6. Delete everything in this folder
   
7. Go into your friend's copy's folder and find the Underworld Ascendant/UA_Data/Plugins folder
   
8. Copy everything in this folder into the folder that you just deleted all that stuff from
   
9. Boot the game, load your savegame
   
10. Check the storage chest at the Midnight Forum, it should have all of your friend's goodies
    
11. Save the game
    
12. Optionally you can now choose to delete the stuff from the steam copy's Plugins folder again and then verify your game cache to get back to a pure steam install, but with the added items. As long as you loaded the savegame and saved again once you had the items I don't think it matters.
    
13. Enjoy having the only backpack item in the game while you think about why you purchased this game instead of stealing it

Alternatively you might try your luck with the console commands, but without a full item list to reference you'll be going in blind. You can enable the console with -console in the launch parameters through your Steam Library. The furthest I got was figuring out how to create some items that are actually named in some stray test files that seem to have been left in the install by the devs.
Something like:
Will put a sword in your inventory, assuming you have first enabled cheats with:
Not sure if messing with this stuff disables Steam achievements or anything, your mileage may vary.

Update:
Cy in the comments below provided a way to get the backpack with console commands.

Originally posted by Cy:

1. in Steam: right-click game title > Properties... > Launch Options and paste in:
   
   %command% -console
2. in game: start a new campaign or load save
   
3. press grave ` (the key above Tab and below Esc ) - a console will open with a small input box in the lower left corner (input is barely visible in 4K)
   
4. put in 3 commands:
   
   cheats 1 create item pr_adventurersbackpack 1 cheats 0
5. bonus tip: while cheats are on, you can press tilde ~ (the same button as before, but with shift ) to open cheats menu; if you press tilde without cheats 1 command, you'll get an error message: Cheats not enabled