One of the (many) great things about this game is the number of Easter eggs it contains. These come in the form of little sentences the mentor occasionally says.

For those who haven’t played the game before, the mentor is a dark, brooding voice that instructs you on how to dominate the world with your evil ways. Generally he simply warns you of danger, “your dungeon has been breached” or domestic troubles, “a Dark Angel is angry because it has no food.” He also briefs and debriefs you in campaign mode.

These are the ones I have encountered, it is possible there are more waiting to be heard. (The reasons for their occurring may not be entirely accurate. /msg me with corrections)

Caused by your actions

"One of your imps does a great Impression of you, he can even do the ears." (Heard when creating a lot of imps)
"The drink is flowing freely in the casino." (Heard when you build a very large casino.)
"Miniature piglets are roaming your dungeon, beware." (Heard when you have a significant rat infestation, therefore a good prison.)
"Big monsters! Big prizes! I love it!" (Heard when you build a large casino.)
"Keeper, you have something unpleasant under your fingernai." (Heard when holding a creature for a long time.)
"Hello? Are you still there? The imps are about to lock up!" (Heard when you leave the computer alone for a long time without pausing.)
"Your dungeon floor is lumpy, order your minions to jump up and down." (Heard when your dungeon borders a lot of unclaimed land)

Compliments

"Your Creatures are in awe of your talents." (Heard when doing very well.)
"Your dungeon is damp, install central heating." (Heard when your dungeon backs onto a lot of water.)
"Your creatures rate you as their all time favourite keeper." (Heard when your creatures have all been happy for a while.)
"Your creatures are happy with your dungeon." (Heard when you build a nice dungeon.)
"Let’s party like it’s 1999!" (Heard when all creatures are in the casino.)
"Your creatures are pleased with your dungeon’s efficiency." (Heard when you convert a lot of heroes in a short space of time.)
"Your creatures are complementing you on your dungeon." (Heard when you build a lot of facilities.)
"Your creatures are impressed with your work." (Heard when you get a lot done quickly.)
"Your creatures like your style." (Heard when you do something brutally and ironically)
"Your dungeon smells putrid, have your imps spray Lysol." (Heard when you have a large number of dead bodies on your hands.)
(My personal favourite)"You have many Mistresses, there's a name for Keepers like you!" (Heard when you have many Mistresses in your dungeon)

Caused by time.

"The very rock yawns with anticipation of your next fascinating move." (Heard when you pause the game for a long time).
"It is the Witching Hour, curses come at half price!"(Heard at 12:00 midnight)
"Surely, even Dungeon Keepers must retire to a lair of some description." (Heard at 2:00 am)
"Your nocturnal perseverance has earned you a hidden gaming tip: go to bed!" (Heard at 3:00 am)
"You know that low, broad, downy-soft item of furniture in the next room? It has the power to cure fatigue and restore vitality." (heard 4:00 am)

Criticisms

"Your dungeon is on an incline, angry creatures cannot play marbles." (Heard when you have no casino and unhappy creatures.)
"Hungry demons cannot reach their toes." (Heard when you have a small hatchery and many Bile Demons.)
"A neutral creature sends word he cannot make it today." (Heard when a neutral creature on the map has been waiting a long time.)
"Your creatures have noticed your evil actions." (Heard when you do not hide the creature you are slapping to get your money back.)
"If you were any kind of real keeper you would’ve won by now!" (Heard when you take for ever to complete a level).

Miscellaneous (God Knows)

"Never eat anything bigger than your own head, Keeper." (Heard randomly it seems)
"Your lair has been re-carpeted." (Might have something to do with a large lair, might be random, I’m not sure.)
"Your minions demand cable." (Not sure why this is heard, could be when you have a very basic dungeon)
"A lost soul has entered your dungeon. Oh, now it’s wandered out again." (No idea, only heard it once)
"Your dungeon is full of yoghurt." (O…k…?)




Jurph says Don't forget that the casino plays "Disco Inferno" whenever a vampire hits the jackpot in the casino (IIRC, there's a way to make this happen more often).

belgand says I would mention that the "Big monster! Big prizes! I love it!" is a reference to "Big money! Big prizes! I love it!" line from Smash TV.