Monsterween: Sewer Sirens
D&D always makes the assumption that most adventures take place in far-flung wilderness and forgotten ruins. While this makes a sort of narrative and simulationist sense, it's a crying shame, because city visits can form such interesting chapters in a campaign. I whipped these horrors up for my players while they were held over in the most populous city in our campaign world Alluvia. They were poking around someplace every bit as dangerous as a classic dungeon: The city's sewers. Sometimes, to be a hero, you have to navigate sh!tty situations.
Polluted Nymphs. The same principle that begets a nymph from a river creates sewer sirens from the numerous foul and forgotten waterways beneath a city. They are much like the waters they are bonded to: Twisting, deceptive, dangerous, and ultimately toxic. Some very powerful sewer sirens may, in fact, have once been nymphs of the river that flows through a city's heart, and are vengefully malevolent against the industries that ruined them.
Junk Collectors. A sewer siren is trapped beneath the bustling heart of its environment, and isolated from the positive aspects of the environment which spawned it. They will often combat their loneliness by befriending rats, thieves, or corpses that find their way into their channels. They are also obsessed with preserving any scrap of beauty that passes into their disgusting lives, and are known to hoard lost coins, wedding rings, and other valuables they can encounter.