The setting for a series of science fiction novels by Jack Chalker, beginning with Midnight at the Well of Souls in 1977. The Well World is a planet-sized computer created by an ancient race that used it to expand and stabilize the universe; its surface is a series of 1560 hexagonal laboratories used to develop new sentient species with carbon-based lifeforms in the southern hemisphere and truly alien lifeforms in the north. Newly arrived space travellers are incorporated into the experiment by being automatically assigned to a "hex" and transformed into its dominant species. Control the Well World and you can theoretically control the entire universe. Of course, it's never that simple.
The Well World series employs the following tropes:
Action Survivor: Nathan Brazil. Despite being the immortal guardian of the universe, he's small, unimposing, and doesn't normally go looking for trouble. His survival is guaranteed but his integrity is not and he's been mutilated quite a bit over the years, and while the stuff he loses always grows back it's a long and painful process.
He only seems small in the modern era. He's only slightly less than average sized for a paleolithic human, and most of human history was spent in the paleolithic era.
All Myths Are True: Centaurs, fairies, fawns are real alien races. Earth was created in seven (subjective) days. Nathan Brazil was Judas Iscariot, as well as the inspiration for the Wandering Jew.
And I Must Scream: The final fate of Juan Campos, the South American rapist drug lord from the Watchers at the Well trilogy. Nathan Brazil decides a fitting fate for Campos is to be dropped into the body of a gorgeous, mute South American Indian woman, who has wandered a bit too close to the Campos compound around the time that the Family's henchmen are looking for some fun. To add squick to the matter, Brazil's Modus Operandi is to find people who have recently died, and bring them back to life with the soul of the person he's dealing with. Campos, my boy, you are now a zombie sex slave for the family you were once groomed to lead. And only you will ever know or believe it. Sleep well.
The fate of a bad guy in the first book. He's dropped into the body of a deer-like animal on the home planet of a species of humanoid which hunts said animal in packs and eats it alive. Wanting to avoid such a grisly fate, he hurls himself off a cliff...only to wake up in the body of another deer. Brazil made sure he will experience death repeatedly, once for every person whose death he is responsible for. He has a fair body count to his credit. And the worst part: he doesn't know that it will, eventually, end.
Author Appeal - Even a casual survey of Chalker's work will show that he was obsessed with the idea of humans changing their species and sex. The Well World books just gave him a justification for doing it on a massive scale.
Not to mention the recurring cases of strong-smart-woman-turned-virtually-helpless-by-vision-or-reading-impairment-and-body-morphing. Seems to happen in the majority of Chalker's books.
Charles Atlas Superpower - Gypsy (Dr. Zinder) has learned to directly manipulate minor local aspects of the universal programming, without technological assistance — he claims it's all done mentally using mathematics.
Enforced Technology Levels - Each hex has a maximum technology level — Non, Semi, or High — to simulate the availability of resources in that race's target biosphere. Technology above a hex's maximum level will simply fail to work inside that hex.
The Fair Folk: An insectoid race that can abuse physical laws by their outstanding psychical powers, essentially making them magical.
Fate Worse Than Death: Nikki Zinder is one of the founders of the Olympians, the race that Ben Yulin created through Obie's powers. When the other founders figure out they're all reaching their age limits, they go back to the Well World for one last life time. Nikki, on the other hand, uses cybernetics to stay alive and becomes a brain in the jar to continue leading her people. When you first see her in The Return of Nathan Brazil, she seems relatively sane until Mavra leaves the room. Then she goes on a religious rant and orders her people to make sure Brazil doesn't fall into the "Evil One's" hands, before laughing madly and then begging in a child's voice for her father to come back to her, despite the fact that she can't reasonably think he's still alive.
He is. He has to Mercy Kill her or her insanity will get in the way of saving the universe.
Fisher Kingdom - entering the Well World induces transformation into one of that world's indigenous races.
Gender Rarity Value - the Olympians are a subspecies of Transhuman Aliens specifically created to fulfill a male villain's harem fantasies — fantasies that backfired spectacularly.
The Fog of Ages - In the first book, Nathan Brazil is so old that he's forgotten most of his life, including his own origins; it all comes flooding back when he comes (back) to the Well World.
The Great Politics Mess-Up - later revised as the universe is rebooted. In earlier Well World stories, communism spread to the stars and there were even people who worshipped Marx and Lenin. After the universe was rebooted, the stories which (partially) took place on modern Earth noted that things are different.
Hermaphrodite - several species, mostly aquatic, as well as variant humans.
Humans Are White: Averted with Nathan Brazil, who is described as having a dark complexion, black hair, brown eyes, and a Roman (aquiline) nose. He's Semitic and he looks it, because he has to look like someone who wouldn't be too out of place in the Mesopotamian "cradle of civilization".
Also with the secondary protagonist, who is Asian (she looks Chinese or Korean). That is, however, the product of plastic surgery to hide her from those who would harm her birth family. At one point she is transformed on the Well World into a form she thinks might look like what she would have if she had grown up normally, and it is blond and busty. So... trope Averted and played Straight at the same time then?
Multiple Choice Past: Brazil has at varous time claimed to be God, the last member of a race of Precursors, or an ordinary human who was tricked into becoming the caretaker of the universe. It's actually the second one.
No Sex Allowed- Taken to extremes by the Comworlders, most of whom are genetically or biochemically engineered to never hit puberty.
One-Gender Race - the Czillians, a species of sentient, parthenogenic plants
Power Perversion Potential - Obie's a supercomputer hooked up to a Matter/Energy/Matter converter and planted inside a planetoid big enough to house a small town. He can give you anything you want, as long as he knows how to make it, and has a big transmitter capable of altering entire planets to a person's specifications. He's also capable of travelling to anywhere he knows the coordinates of within minutes. Upon hearing of Obie's full power, Nathan Brazil himself said, "to hell with porn," and upon seeing Obie in action, Brazil was rather disturbed.
Ragnarok-Proofing - The Well World itself. Not only is it a planet sized computer that has been functioning for millions of years since its creators dissapeared. but if true ragnarok is at hand you can just ctrl-alt-del the thing and reboot the entire universe
Starfish Aliens: Well World is divided into two parts. Southern Hemisphere is for carbon-based lifeforms, and a lot of those are quite starfish-looking. Northern Hemisphere is for non-carbon-based ones, and those are just plain weird.
Steam Punk - in certain hexes, technology is limited to steam.
Wandering Jew: Brazil again, and he's been wandering for at least a billion years.
What If God Was One of Us? - Maybe, but also practically yes. Nathan Brazil's real origin is never stated, and several of the characters wonder about him: is he really God, the last Markovian, or a construct of the Well itself? At the end of the day, it doesn't matter because once he's in the Well Computer, he is a deity. The last book reveals he is indeed the last Markovian.