Modern source ports (e.g., gzdoom, ZDoom-derived ports, or Chocolate Doom for classic behavior) require the original HERETIC.WAD to run. Steps:

The most reliable way to get the legal, full-version WAD is to purchase the digital edition of Heretic: Shadow of the Serpent Riders

To play the full version of , you need the main internal data file known as HERETIC.WAD . While a shareware version ( HERETIC1.WAD ) containing only the first nine levels is widely available for free, the complete "Shadow of the Serpent Riders" expansion must be purchased legally to obtain the full 5-episode WAD. Where to Buy the Full Game

A compilation of over 400 WADs available on Mod DB.