HEXDD.WAD is the IWAD used by Hexen: Deathkings of the Dark Citadel. While it is technically an IWAD, the only data in the WAD file itself are the extra levels (and their associated lumps), and some small graphics and end message changes, so the game will not run without an install of Hexen.
Version 1.1 is 4,440,584 bytes in size, is dated 1996-05-09, and contains 326 entries. It has the following hashes:
Compared to version 1.1, the missing entry is the SNDINFO lump, which is used to assign MIDI music track to the maps. Its absence means that no music is played unless audio CD music is used (since CD tracks are associated to maps in MAPINFO instead). This is the only difference; all other lumps are perfectly identical between both versions.
At the time of writing, version 1.0 is the version distributed via Steam, alongside Hexen version 1.1.