Not that familiar with unity, but only action is most likely to take them to court. Disgusting that someone does this. If i remember right steam takes these things very seriously, most likely you can get the game removed from there without much of an issue. After all they are selling it for 4 euros and claiming its their game. Contact a lawyer and see what happens
The name of this asset is Horror FPS Kit and anyone who buys this asset owns the copyrights of that asset. Do not pretend to be the owner of the package. The package owner made that aisle, not you. We wanted to contact you about this issue, but you didn't even respond? What is your purpose? You claimed copyright as if the artwork was yours. This is a false statement.