"In Ptolus, the supernatural is expected and treachery lies around every corner—or is it that the supernatural lies around every corner and treachery is expected?"
“Some place needs to be the worst place in the world. Why not here?”'
— Brusselt Airmol, well-known rogue adventurer
Ptolus: Monte Cook's City by the Spire is, in his terms, the most deluxe campaign setting ever. Almost seven hundred pages long, not including the many other materials that came with it as PDFs on a CD inside the cover, it's a massive, detailed view of a world, and in particular one city, that runs on the D&D rules. The author was one of the creators of Third Edition D&D, and this was one of the last major products before Fourth Edition.
This book/setting provides examples of:
Adventure-Friendly World: Played with. The world at large is noted to be not particularly friendly to adventurers, who are viewed as oddballs at best and wandering psychopaths at worst. But the main setting, the city of Ptolus itself, is having a kind of adventurer gold rush. Not everyone loves the adventurers, but everyone is used to them, and the economy has accommodated the influx.
Aristocrats Are Evil: Mixed. Some aristocrats are nearly monsters (House Vladaam). Some are evil but manage to play well with others (House Sadar). Others are fairly benevolent (House Dallimothan), and Commissar Igor Urnst, the ruler of the city itself is, if not good, then decent and highly competent.
Beneath the Earth: There are many different, intersecting sets of natural and artificial caverns under Ptolus, from the sewers to Ghul's Labyrinth to, if you go deep enough, the cities of dark elves.
Big Bad: Over thousands of years of history, there have been many... and they've tended to end up in Ptolus. The Dread One, Eslathagos Malkith, built his fortress atop the spire. Later, the so-called Half God Ghul built his fortress halfway up. And deep below the city the Galchutt wait for their chance to destroy everything.
Body Horror: This is one of the things chaositechnicians specialize in. Kinion Luth, the Surgeon in the Shadows, is the prime example
City of Adventure: The city is built around Sealed Evil in a Can and on top of multiple layers of Sealed Evil in a Can, and (mostly unrelatedly) is home to several men who are, or can at least get away with claiming to be, emperor. The evil is leaking, the cans have become something of a tourist industry, and the political tensions are on the rise. Yes, there are some explanations.
The book itself gives the rest of the world only in the sketchiest details. And it suggests that GM can agree with players that the focus is just never going to leave. But with all that's going on, why would you?
Cool Sword: Plenty to go around. The six hungerswords, most of which are missing, and the Sword of Lies are noteworthy.
Eldritch Abomination: The Galchutt are very much homages to Lovecraft. Monte Cook is a fan and wrote Call of Cthulhu D20 and the Shandler Chronicles, a Mythos serial.
Elves VS Dwarves: Subverted. Elves and dwarves are quite different and usually aren't buddies, but they have nothing against each other and no one is surprised by a mismatched pair.
Entropy and Chaos Magic: The Dread One's Entropy Sphere. Ghul tapped it and created Pits of Insanity. And, of course, chaositech and chaos mages.
Evil Tower Of Ominous Ness: Ptolus lies in the shadow of the impossibly tall Spire. Though not many people in the city realize it, the entire spire is hollow and holds a vault of evil artifacts, and on top of that is the castle so tainted by its former Big Bad occupant that the gods themselves still keep it locked tight thousands of years after his death.
Halfway up the Spire is the fortress of a slightly smaller bad, Ghul. He plunged most of a continent into winter for years as a weapon of mass destruction, created monstrous laboratories in which to create monstrous armies, and generally was bad news for everyone and everything. And he measures up to the halfway point of the original big bad.
Fantasy Kitchen Sink: By design. Monte Cook thought many settings failed to take into account the effects that magic and high level characters would have on a world. Ptolus is a place where the city guard invisibly monitors for mind control in markets, you can buy resurrection insurance, get Magitek implants, and look at both ancient and new steampunk devices.
Fantastic Racism: It is illegal, on penalty of death, to be a dark elf within the city limits.
Left-Justified Fantasy Map: Not really. The map has land running off the west and east sides, with sea below. It looks something like a distorted map of Western Europe, with the Prustan Peninsula as Italy, Ren Tehoth as Germany/France, and Palastan, Cherubar, and the Sea Kingdoms as the Iberian Peninsula. Qurac across the Southern Sea is like Morocco.
Necromantic: The lichloved, a cabal that even the rest of the Forsaken find icky.
Omnicidal Maniac: Eslathagos Malkith. Ghul, too: he turned off the sun for years.
Our Angels andDemons are different: Because Praemal is a prison plane, extraplanar beings can enter but then they can never leave. Some angels still come if they believe they must, to do some good. They tend to congregate in the Ivory Tower in Ptolus and are collectively called the Malkuth. The demons who arrive in Ptolus for various reasons, including those who first came with the Galchutt, hang out in the Necropolis as the Fallen, and their non-evil fallen angel leader considers whether or not to free the Galchutt just to get out. Plus there are the native earthbound demons.
Our Elves Are Better: Shoal elves, harrowed elves, and winged cherubim elves. The Urthon Aedar, too.
Our Orcs Are Different: Like the Tolkien version, most were created by Ghul as modifications of another race. Except he modified the Ornu-Nom orks to create the more vicious and evil Toruk-Rul and Sorn-Ulth. The originals are less monstrous... but still enemies of most other races.
Our Trolls Are Different: The city watch is willing to pay very well for people willing to be polymorphed into trolls. They don't get many takers.
Religion of Evil: Chaos Cultists. Worship of Destor, banned by law. The Forsaken.
Saintly Church: The Church of Lothian is political and has made its mistakes and enemies, but it's a lawful good church and its members genuinely try to do good. Other churches to more minor good deities as well.
Science Is Bad: Subverted. Chaositech looks pretty much like science fiction and is The Corruption...except it isn't science, and it isn't magic, it's taking advantage of flexible reality produced by The Corruption to make things. In reality, true science, as represented by the Shuul, is morally neutral, and part of the recommended adaptation of chaositech to one's own setting is to contrast it's corruptive, primordial nature with the real progress of actual technology.
Sealed Evil in a Can: The entire world of Praemal was created as a prison to keep the Galchutt from destroying all worlds everywhere. Its prison status means everyone else is sealed in the can with them, but at least they've got several more cans around them.
Danar Rotansin decided to gather up all the evil artifacts he could and seal them in the Banewarrens so nobody could use them. This had some side effects, but even when he himself had a Face-Heel Turn most of the contents remained sealed away and undisturbed for millennia.
Smug Snake: Shilukar. In a campaign setting with immensely powerful, monstrous foes, this dark elf tends to get the most intense hatred from players. And he often gets away with his schemes anyway.
Staying Alive: There are a lot of things in the Banewarrens, Goth Gulgamel, and Jabel Shammar that aren't alive, exactly, but aren't quite fully dead either.
Time Abyss: The cthorn in Goth Gulgamel. They're undead, but they're the last of a species that has been forgotten for a very long time. The point is for players to fight them and then wonder what the hell they are and then consider just how long they've been lurking.
Tome of Eldritch Lore: The Book of Inverted Darkness is a major one, but wizards sell minor tomes by the hundreds.
The Undead: They abound. The Necropolis is full of them, and the Wintersouled, the first undead from when the veil between life and death was torn ages ago, still lurk quietly.
Weird Trade Union: The Inverted Pyramid, a kind of mage's guild, runs the Dreaming Apothecary. Want to purchase a magic item? Get a token, go to sleep, and they'll arrange it all in your dreams. They'll also arrange to blow up your house if you try to become competition.
Well-Intentioned Extremist: Helmut Ittlestein, leader of the republican movement. Actually fairly reasonable politically, but willing to have his lover murdered because he believes she is pregnant with a child prophesied to bring doom to the world. He's a villain many players have mixed feelings about.