Gutter Fey

As humanity moved into towns that grew into huge cities, many of the fey that existed alongside them adapted and changed rather than departing back into the feywild. These fey eventually developed into city fey or, as they're commonly called, 'gutter fey'. All around cities, sneaking along eaves in buildings, hiding among the refuse in alleys and sliding along gutters on the edges of roofs, these fey act as they did in the past; they clean up things when treated well, break things when treated poorly and, above all else, engage in mischief both petty & cruel depending on their nature.
Since fey creatures are only partially material beings, they can slowly shift in appearance & demeanor to reflect their surroundings and how they feel. In short, fey that are kept happy and are treated well, tend to be pleasant and those that are abused or otherwise treated badly tend to devolve into vicious beasties. This is particularly dangerous around Saether-equipment which some fey (commonly called gremlins) take great delight in disassembling or sabotaging. In fact, in the year 884, a group of gremlins that had been treated poorly in the city of Korth meddled in the cities Saether-reactor, causing a chain of system failures that culminated in a series of explosions that caused immense damage to the city from which it still has yet to recover.
In some cities mistreatment of fey is a crime (to varying extents) where-as others simply have strong taboos against. Many areas have different traditions about how best to treat fey.