Hidden Features:

See messages from the game's programmers:
Hold down the Left Shift button and type FECK OFF or SKEKSIS. The messages will disappear if you release the Left Shift button.
Start the game with the command line parameter -alex. Then while playing hold down both Shift keys and type JLW to see the message.

Get a Dark Mistress:
Sacrifice a Spider and a Beetle in your temple.

Get a Bile Demon:
Sacrifice 3 Spiders in your temple.

Get the Horned Reaper:
Sacrifice a Bile Demon, a Mistress and a Troll in your temple.

Turn all monsters into chickens:
Sacrifice a Mistters and 2 Bile Demons in your temple.

Explode all your chickens:
Sacrifice one chicken in your temple.

Lower Imp Cost:
Sacrifice an Imp to lower the price of an Imp by $300.