Hexen: Beyond Heretic

Wikipedia
https://en.wikipedia.org/wiki/Hexen:_Beyond_Heretic

MobyGames
http://www.mobygames.com/game/hexen-beyond-heretic

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

https://doomwiki.org/wiki/Hexen_source_code

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