Released in 1997, Hellfire was an "unofficial" authorized expansion. Because it wasn't made by Blizzard North, it isn't considered canon. Key features include:

Two new dungeon tilesets—the Hive (insect-themed) and the Crypt (undead-themed).

The Hellfire expansion, developed by Synergistic Software, technically only went up to v1.01 officially.

Finding a "free download" for usually refers to one of two things: the original game files (often hosted on "abandonware" sites) or the official v1.09 patch for the base game. 1. Legal Status & Availability

The Monk , a melee fighter who excels with staves and bare-handed combat.

You can purchase Diablo + Hellfire on GOG.com. When you buy the original Diablo there, the Hellfire expansion is included as a free bonus.

If you already own the original game, you can download the Diablo v1.09 Patch from ModDB for free to fix Battle.net issues and minor gameplay bugs. 3. What is the Hellfire Expansion?