Just face it, you won't get meaningful replies here unless you're lucky enough to find someone who's reading this and actually studied licensing laws.
My good friend is a lawyer who helped Westeros: Total War with their Cease & Desist letter. It pretty much boils down to this: You use a company's IP, it is within their right to tell you to stop and you pretty much have to do it. The thing is, even if you say no, they can shut down your website by telling your hosts, then the hosts would then have to face the legal challenge and they will most likely just turn you off, less of a loss. Your forum has advertisements on it? You are profiting from the IP by people visiting your thread in regards to that mod.
The only way you can win is by compromise or reaching an agreement with the company involved.
US case-law also has a lack of legal precedents when it comes to this field as well, because simply no one is stupid enough to pay thousands of dollars in legal and lawyer fees to challenge an IP holder over making a mod. There are a few very good lawyers who would be interested and take on such cases (such as my good friend) but as a realist, it isn't something you want to consider.
This.
Which is why I was advising the Diablo modder to actually try to turn his work into something original where no issues will arise.
Any fan who is a true fan of something will have the decency to ask permission from the author it's called: Respect. If the author/owner declines you just accept it, you don't waste your time, and the owner stays happy. Both parties win.
I see only one purpose for someone to completely ignore this do a mod based on another persons work/property, it's because they can't come up with anything original or good enough so they decide to ride off the fame or quality of another's work.
It's like one comic book artist copying the work of another comic books artist, using the same characters, background, and theme, then creating future storyline and releasing it to the public for free. It's plain wrong. Illegal, and lowers the reputation of the copying artist.
Unless they have of course come to an agreement where both parties are happy then no one can stand in their way.
