Instructions
Locate the "Diablo: Hellfire" directory on your computer. The default directory the game installs to is "C:\sierra\hellfire." If you cannot find this folder, you may have specified a different path during the installation process.
Download the "hellfire-all.zip" folder to your computer if running "Diablo: Hellfire" version 1.01. Obtain "hellfire-all.zip" from the Incgamer's website. If running "Diablo: Hellfire" version 1.00, then you can already access the game's online component so skip to step eight.
Unzip the compressed folder you downloaded using the file compression utility of your choice. Do not extract this folder within your "Diablo: Hellfire" directory. This will add extraneous files to your game folder.
Open the "hellfire-all" folder and view its contents. You should see the following files: "hf101.exe," "Hellfrui.dll," "command.txt," and "info.txt." Read the instructions contained in "info.txt" and remove the file from your computer.
Either install or delete "hf101.exe." This file is the official patch that upgrades "Diablo: Hellfire" to version 1.01 and removes all multiplayer options. If online play is disabled in your game, then you already have this patch installed.
Replace the "Hellfrui.dll" file in your "Diablo: Hellfire" directory with the newly-downloaded version. Click and drag (or cut and paste) the "Hellfrui.dll" file from the "hellfire-all" folder into the game directory (C:\sierra\hellfire by default). Click "Yes" when your operating system asks you to confirm if you would like to replace the file.
Transfer the "command.txt" file to C:\sierra\hellfire (or the directory path you specified during the game installation process). This is the last step necessary to enable online play in "Diablo: Hellfire" version 1.01. With both files in place, you can boot up the game and access the previously disabled online multiplayer menu.
Connect with friends for online play. "Diablo: Hellfire" is not compatible with Blizzard Entertainment's popular Battle.net service, but you can coordinate games with friends or other "Diablo" enthusiasts and play over a direct connection. Choose "Multi Player" from the game's main menu (this will only appear in version 1.00 or the hacked version of 1.01) and select the appropriate option to either host or join an online session.