General

An intelligent parasite that lurks in shallow, muddy waters and leaf-littered river bottoms, the Yggdrid Leech, or Mana Leech as it is often called, is an eel like creature no longer than a child's forearm, that latches onto its prey and feeds, as per any parasite, on warm mammalian blood. They are instinct driven, but not particularly aggressive in normal situations.

However...

Due to an impressive array of antennae and sensory apperatures, these little worms can become actively aggressive, eschewing all others, to swarm upon and attack a spell caster. Their frantic activity can make the water seem as if it is boiling, when a magic user enters their purview. Their entire intent, the whole of their existence, becomes the desire to latch onto the creature and take control of their nervous system, much like a Roach Wasp.

A character so dominated becomes a passenger in their own body as the Leech, with an almost childlike curiosity, approaches the world as its own personality and with its own incomprehensible agenda. It could be obsessed with providing education to underpriveleged children, just as easily as it could become inconsolably kleptomaniacal. They become obsessed with experiencing new things, and gaining new knowledge. Typically, all of this curiosity is sated in perfectly benign, if unusual, ways, although old friends of a magic user so afflicted would notice a personality change immediately.

Occasionally other Yggdrid Leeches will latch onto an already dominated host, forming a small collective that will work together to perform some random task that makes sense only to themsuch as randomly building a cuppola in the middle of nowehere, or studying food service and cookery until they are able to construct the perfect Beef Wellington.