This was originally intended to be an expansion pack of Heretic (1994) but was eventually released as a spin off, not an expansion pack.
There is a cheat code in the game named 'DELIVERANCE', which briefly turns the player into a pig, and displays the message "Squeal!". This is a reference to the movie Deliverance (1972)
The symbols on the green and blue mana are actually Japanese hiragana and katakana (phonetic symbols). The blue mana is a hiragana "to", the green mana a katakana "su".