You can follow the above links to get more info about these but in a nut shell the challenge is to create your own version of New Crobuzon (from China Mieville's book). Quoting Anne," There are two parts to the challenge - first, choose 3 humanoid monsters to be minority citizens in your fantasy city and assign them cultural niches; second, choose 3 horrible monsters and decide how the city accommodates their presence without them destroying everything."
I love the Fiend Folio and so I decided to use it heavily as a resource for this challenge.
I picked Githzerai, Tabaxi and Grimlocks as my minority citizenry and Grell, Blindheim and Clubneks for my monstrosities.
Citizens- Githzerai, Extra-Planar Mercenaries
The githzerai started arriving in New Jericho in small groups several decades ago. They keep mostly to themselves and maintain a low profile. Their exact numbers in the city are unclear, always only being encountered in small groups, however, many believe there are far more present in the city than anyone realizes. They can be contracted as mercenaries for expeditions into the Sunken Quarter, but their services aren't cheap, and it is whispered by a few of those who have interacted with them that the githzerai seem to be looking for something, something lost and forgotten by the other races.
Grimlock from Fiend FolioCitizens- Grimlocks, Underclass Riff Raff and Thugs
The grimlocks are disliked and looked down upon by most everyone else living in the city. Believed to be the degenerate descendants of the founders of one of the older cities New Jericho was built upon. They lurk in the pitch black tunnels and forgotten chambers running throughout the Sunken Quarter beneath New Jericho. The grimlocks are divided into different clans and tribes, some seeking to live in peace with the surface dwellers and even try to trade (bringing an extra bit of lost treasure to the surface), while others fight among themselves for food, territory and resources, while others attack the surface dwellers whenever they get the opportunity and think they can get away with it. On occasion a spoiled noble youth from the city will campaign for grimlock equal rights, more to annoy their noble parents than anything else, most of these youth either end up bored and move to other pursuits or get eaten by the grimlocks. If the grimlocks could ever unite under a strong leader the other citizens of New Jericho would be in trouble.
Citizens- Tabaxi, Valued Servant Soldiers
For generations the Tabaxi have served the ruling nobles of New Jericho, originally discovered in the distant Kudzu Jungles of the South, their extraordinary skill in hunting prey and aptitude with weapons made them a perfect choice for the elite of New Jericho to use in their wars. Over the generations many have been brought to New Jericho and have proven themselves useful for the ongoing expeditions into the Sunken Quarter and even darker places.
Monstrosity- Grell, Otherworldly Intruders
The Sunken Quarter hides denizens far worse than the grimlocks. The grell are one of the more frightening dwellers in the dark halls and ruined corridors beneath New Jericho. The grell are said to have been the cause of the destruction of one of the prior cities that New Jericho was built upon. It is believed that in ancient times a portal was accidentally(?) opened to the domain of the grell and a horde of them were able to pour through and ravage the city, wrecking mass havoc and death before the portal could be closed. Only the dawn of the sun allowed a few desperate people to survive, while the grell retreated to the dark places below ground. Although otherworldly, the grell are intelligent, and have the means to work together, using tactics and skill. They have thus far shown no interest in seeking to engage in politics, overtly attack the surface or even attempt to dominate the grimlocks to any real degree. They will devour the grimlocks just as readily as any human, tabaxi or githzerai they encounter. It is said that some grimlock tribes will capture humans and other races to give to the grell as tribute so the grell will leave them alone, and those that can't offer up appropriate tribute will sacrifice their own tribe members to appease the grell. In some whispered circles there are those that fear the grell are up to something...but what could it be??
Monstrosity- Blindheim, Domesticated Degenerates
The exact origins of this frog-like subterranean humanoid creatures aren't clear, but many sages believe they are another of the degenerate descendants of earlier inhabitants of one of the older cities New Jericho is built upon. Irregardless, in the Blindheim they have found a tool to help them explore (reconquer) The Sunken Quarter. Using the creatures innate "searchlight" ability, expeditions sent into the Sunken Quarter have "domesticated" these creatures and through use of a control harness and leash are able to use the blindheim as portable, trained, living searchlights. This enables explorers to spot grimlocks in hiding, grell ambushes, or hidden recesses and alcoves. While the blinding light is useless as an attack against the grimlocks and grell, nonetheless expeditions have found having one or two on hand to be a helpful tool in their explorations, as long as, they can keep from the beast turning on them and blinding them in an attempt to escape.
Clubnek - art from White Dwarf #9
Monstrosity- Clubnek, Cavalry Beast
While New Jericho does have an extensive Sunken Quarter that consists of layers upon layers of ruins from prior ages, not all of the city's interests lay underground. The rulers of New Jericho maintain an effective cavalry force of clubnek riders to help advance the interests of this city well beyond its walls. Generations of careful selective breeding have led to the development of a fearsome cavalry mount that when combined with a skilled rider can turn the tide of many a battle. Wealthy nobles have even gone so far as to have bred clubneks of exotically colored plumage. Clubneks used by the city's military have had their natural green plumage selective bred down to a light brown color to enable more discreet operations in the wild.