Mageripper

Magerippers were tiny, utterly bizarre aberrations that both sensed and fed on magic, much to the detriment of any spellcasters in their vicinity.

Description
On average, adult magerippers were only long and weighed. They were eyeless, instead have circular discolorations marking the location of specialized magic-sensing organs. The legs were insect-like, the two tentacles vaguely like those of a displacer beast, and the body was conical with oversized, sharp-toothed jaws. Magerippers were sickly purple, blue, and/or pink in color.

Behavior
Magerippers were animalistic, solely concerned with finding food and breeding. They weren't intentionally destructive but they were highly erratic.

Abilities
Magerippers were blind, but they could innately sense any magic auras within of the swarm, and just as naturally could detect the strength and nature of the auras, as well as any use of spells or spell-like abilities. When the swarm entered the same space, it could feed on an ongoing spell or spell-like effect, effectively dispelling it. More passively, magerippers swarms drained magic out of other creatures, reducing their magical abilities over time.

Combat
Mageripper swarms were relentless in pursuit of food, always heading for the strongest magic aura they could sense and surrounded the source until it was drained or destroyed.

Ecology
Magerippers lived in temperate climes and were drawn to places with strong and plentiful sources of magical energy, typically urban areas or regions home to many fey and/or elves. Beings inimical to magic were known to try and encourage magerippers to settle in their area, but the creatures, while effective guardians, were prone to leaving in search of more magiferous feeding grounds; other uses included using them as a defense against magical assaults. "Keepers" of swarms were known to cast a spell on a captive, or binding a spellcaster, and leave them to be killed by the hungry swarm.

Technically, magerippers could survive by eating meat, but they thrived on magical energy and required it to reproduce. Magerippers reproduced via asexual budding. With enough magic, a swarm could double its numbers within days; in such instances the swarm split into two that went their separate ways. Individual magerippers only lived one year, and grew sluggish, unresponsive, and eventually dead if separated from their swarm.

They could not feed on the magic in magical items, but still swarmed around such treasures.

Mageripper swarms were among the creatures found in the sewers of Waterdeep, and they had also been reported living in Skullport. Additionally, the Thayan enclaves established by the Red Wizards often contained many magic items, making such places magnets for mageripper swarms, and keeping the swarms under control was a constant concern of the Thayans.

History
Mageripper swarms were so bizarre that most scholars theorized that they had been deliberately created for some unknown purpose.