These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Demonic Spiders: Each level has a monster that's automatically the strongest of the bunch, capable of holding off and even killing heroes, and their pentacle can only summon one of them. Said monsters are the Guardians in Catacombs, Living Statues in Temple and Fire Wurms in Hell. Ironically enough, Spider Demons are just Elite Mooks.
Game Breaker: Downloadable contents include the Dark Lord Statue that gives you a small but useful boost in all your skills. You can also buy several other machines that drastically improve the services of your Dungeon.
Goddamn Bats: Again, each level has at least one pitifully weak monster that's usually found in large flocks. They include Vampire Bats in Catacombs, Winged Snakes in Temple and Flying Skulls in Hell.
Moral Event Horizon: Torturing or sacrificing prisoners will anger heroes and champions of justice. Sending three or more people in a row to the torture chamber/dark altar is the quickest way to have a Champion running around in your Dungeon.
Most Annoying Sound: Whenever you're attacked, or a hero manages to escape or is slain by a trap or a monster.
That One Level: "Merrycity on Milklake". It starts off pretty well, but you'll have to smash 30 of your prestige machinery (which lowers your current power) and not too much time later you have to fight back and conquer two Dark Lords at the same time coming from different locations on the map.
The first two missions in Hell: in the first one, you have to assist Marthas in his fight against Volbar by invading an enemy area which can't be influenced by your aura full of strong heroes. At the same time, you have to do Kalypso's really annoying sidequest. In the second you have to recover the Sword of Destiny... and deals with hordes of heroes, keeping Marthas alive, paying huge amounts of money to Kalypso and invading an enemy area full of heroes.