HacX (pronounced "hacks") is a first person shooter game. It is a licensed total conversion for Doom II. It was created by Banjo Software in 1997, and featured an entirely new set of graphics. HacX uses a DeHackEd modified version of the Doom engine and the authors of HacX paid id Software $5000 for the rights to market HacX as a commercial add-on for Doom II.
In 2003, the creators and copyright holders of HacX released the game as freeware. The wad file released originally required the official DOOM 2 wad from the commercial Doom II game. As of version 1.2, the game is a total conversion and doesn't require any other game to run.
Alternatively, BlackAura updated this wad file with replacements for the missing parts with pieces from FreeDoom or blank files. His updated HacX file is available for download from the Dreamcast Doom Page.