What aspect of Diabdat.mpq or Diablo 1 would you like to explore further?
The Hellfire expansion (by Sierra, not Blizzard) uses a separate file: hellfire.mpq . However, it still relies on diabdat.mpq for core assets. If you mod diabdat.mpq , Hellfire will also be affected.
: The full version is roughly 500MB . If you have a version around 50MB, you likely have the "Spawn" (demo) version, which lacks the full cinematics and late-game levels.
Why did Blizzard use MPQ?