Me and Xenonaut (Xenonaut cool) have been working for the past four days to make a birthday present for a friend of mine, The Duper Trooper. We chose an asset mod for Helltaker for some god forsaken reason, and I want to talk about how we did it. (Xenonaut did 10% of the sprites, I did everything else)
This is a basic guide on how to change the 2d assets from Helltaker. I have no idea what I'm doing, so take this with a grain of salt and maybe run untrustworthy exes in a vm. There also may be some missing dependencies I forgot to mention.
1. Finding the assets
Obviously, you'll need to get the assets out of the game. Helltaker conveniently has an artbook which we referenced the sprites from. If you don't have the leisure of having the assets, load the .assets files into AssetStudio, a nifty tool to explore a games assets. You can use it to view and extract the assets, and it will help us later to verify that the sprites have been loaded in successfully.
2. Modifying the extracted assets
This is pretty self explanatory. Change the sprites to whatever you want, or take the extracted sprites and draw unibrows everywhere.
3. Loading the sprites back in
Using another tool, Unity Assets Bundle Extractor, we can modify sprites. Install Visual Studio 2010, it's required by UABE to function. Open the .asset folder for the sprite you want, click the plugins button > edit > load texture and select the new image. Click ok, and once you're done with all of the sprites in the file, go to file > save and save it to a location somewhere. Close UABE and AssetStudio, and then you can replace the original file.
4. Testing
Your hard work pays off when you start the game and see Dupers magnificent face (or is that just me 
)
If you want to play Dupertaker, you can download it from this mediafire link.