Heretic

Wikipedia
https://en.wikipedia.org/wiki/Heretic_(video_game)

MobyGames
http://www.mobygames.com/game/heretic

Doom Wiki
https://doomwiki.org/wiki/Heretic

https://doomwiki.org/wiki/Heretic_source_code

SourceForge
https://sourceforge.net/projects/heretic/