Find DIABDAT.MPQ in your original CD directory or your GOG installation folder ( C:\GOG Games\Diablo\ ). Open the Archive: Load the file into the MPQ Editor.
The Hellfire expansion (by Synergistic Software, not Blizzard) introduced its own HELLFIRE.MPQ , but it still depended on DIABDAT.MPQ for core assets. Diablo 1 Diabdat.mpq
(1.09 or Hellfire):