It's super illegal. Irrespective of whether you need a commercial release of the game in order to actually play Diablo with this engine, the fact is that they're distributing a derivative work of software copyrighted and proprietary to Blizzard, which is illegal under copyright law without authorization. And that's not even getting into due diligence w.r.t. other IP concerns; there could be third-party code or trade secrets in there.