Question about Copyrighted material.

[Tesseract]

Of course you can use the assets for a custom story, it's an Amnesia game made on behalf of FG. It would be illegal to take the models and upload them to a site like turbosquid and sell them off as your own. It would also be illegal to take the assets, music or whatever and make your own game with them. If you're using the models or music in any way that makes you money (without permission) you will be in trouble.