Jelly Leech

Hovering in a crystal orb slightly larger than a plump apple are half a dozen dark grey shapes roughly resembling minnows or fry of some manner of sunfish dappled with tiny shimmering scales. On closer inspection this apple's caramel topping is a bronze colored solder sealing the crystal lid, around which a coarse knotted twine holds a waxed parchment label reading "Jelly Leeches. Break only if aquatic plague can be solved with aquatic plague." Sensing your disturbance of their prison, the jelly leeches wiggle too and fro. A nearby bestiary lies open to a page depicting these tiny destroyers:
The Jelly Leech is an unusual member of the order of oozes in that its size is tiny, and that its visual appearance delibeately attracts attention. They are predators to creatures their own size or smaller, and viscious parasite to many that are larger. Specimens can be found in waterways mimicing small fish to entice other predators to swallow them, where they begin the process of consuming the creature from inside out. When the water is calm they can also be found feeding on detrius, plants or dead fish.
Favorably tepid and overcast conditions can result in a population boom, killing everything and ruining the waterway for a year or two while the leeches destroy their own food sources, and fall prey to creatures that have developed resistances to their venom. They are believed to have originated in the murky interconnected wetland waterways of south central Rynabor where favorable temperatures persist, and shelter from strong sunlight is abundant.
Anyone entering infested waters should remember afterward to check their skin closely for leeches, and perform an enema with Kamal's Brine in case of aggressive intrusion. Handling a jelly leech can causes a severe rash and light blistering, with prolonged exposure resulting in temporary paralysis, necrotizing fasciitis, and gangrene.