A villager who lives near an entrance to the underworld might meet a group of adventurers going to explore. Perhaps they have among their number a grim barbarian, a courteous and gentle knight, a scheming wizard, a merry halfling, and a nature-loving elf. How, the rustic might wonder, did such a varied group ever meet, let alone decide to enter danger together? The answer might lie in the custom of this city.
Long ago a drowned sailor's widow grew to hate the sea. She found the largest island she could, and walked inland with an oar over her shoulder. When someone asked her what the oar was she stopped, and founded this city.
Because of her lonely widowhood, the custom of the city has ever been to group orphans and the like into 'families', that they might support and comfort one another. Upon the completion of appropriate rites the group are considered brothers and sisters, in law and custom.
Unfortunately, travellers who enter the city without their family are assumed to be orphans by the city's citizenry, who would not think to do such a thing.
The evil of this custom is compounded by the law that if one member of a family dies without a will, his relatives inherit. How much blood has been spilled in the darkness due to this single law? It surely outranks in infamy even the belief that the sole survivor of an expedition is henceforth blessed with the luck of all who died.
On the other hand, the custom has created a situation where the barbarian of our example, full of fear and hate for all magic, will yet protect his sorcerous 'brother', and not kill him outright, burn the corpse, then sow the burial ground with salt so that nothing will grow (as is the custom among many tribes). The thief will not steal from the priest, nor the knight slay the thief, nor the witch corrupt the knight, nor the priest condemn the witch. This implausible companionship may be found throughout the world, and the underworld, especially near the city.
The traveller to this city is cautioned, then, never to enter an inn in the foreigner's quarter, lest they find themselves sitting at a table with some random strangers, and find that they have "decided to adventure together" in the eyes of the authorities. Better to wake up on the cobbled street than to wake to find all your wealth in the hands of a 'family treasurer', your amorous glances at your new 'sibling' earning you contempt and disgust in the eyes of all, and with a life sentence of defending an evil wizard with body and sword.