Here is what we've found so far:
Imps - Sacrifice a few of these little guys and they get much cheaper to create! Well worth it.
Fly & Spider - Get a Warlock
2 Beetles - Complete current manufacturing
2 Flies - Complete current research
Beetle & Spider - Get a Dark Mistress
3 Spiders - Get a Bile Demon
Ghost - All your chickens die
Vampire - All monsters diseased
2 Bile Demons - All monsters turn into chickens
Horned Reaper - All monsters angry!
Dark Mistress & Bile Demon & Troll - Get the Horned Reaper
Do NOT sacrifice the undead, or chickens, unless you like making the gods angry.
Speaking of the Horned Reaper, he's a great fighter, but can go a bit bezerk at times. If he gives you too much trouble, you can pay him off by dropping money on him. Makes him happier (me too).
You can find specials for access to secret worlds on the following levels: 8, 9, 14, 15, 17
Also, there are levels that only appear during certain phases of the moon (such as the full moon). The program determines this by your CPU clock, which can be changed. So get out the almanac and start exploring!
Solved the game? Can't find a biological opponent? You can play all those multiplayer maps against the computer! Just go to your DOS prompt and start the game using '-1player' as a parameter. In the following example you should be in your Dungeon Keeper directory: