2) Go to The Hell's homepage, Downloads section. And download the mod itself. You'll be able to identify it by the date of release. Also, you have optional downloads: gladiator & assassin class improved looks (GFX downloads) and TH music add-on. In case you're having problems with downloading from Bigupload service, check the Download Mirrors (they support download resume), but check for versions carefully.

3) Put the downloaded files to the directory where you have The Hell installed (I recommend making a copy of Hellfire folder to prevent problems with the original game in the future) and unpack the files. If prompted, choose "overwrite".

4) Copy all of the MPQ files (diabdat.mpq, hellfire.mpq, hfmonk.mpq) from your Hellfire CD to your The Hell directory.

5) Run the game with [ Run_Hell.exe ] file and have fun!
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If you lost your Hellfire CD or it's not in english, you can use this little section for your Diablo Hellfire download, and also get the mod along the way.

Download TheHell.ISO from this torrent link. It includes all needed Diablo & Hellfire files.

Or, you can download Diablo Hellfire (original one) from eMule link below:
ed2k://|file|Diablo.Hellfire.[Szpakos].(osloskop.net).iso|432093184|3F49C63C7166F30138C7CC4276B125A5|
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I f the above didn't help you, report here and provide detailed description of the problem.