This creature lives in the dark southern jungles, but has found its way to the civilised north. Often hiding in fruit trees or on shrubs favoured by local fauna, it bites its host, injecting a strong paralytic poison, and finds a good spot to stay attached for years, which its host often survives. (Note that it is mindless, and acts only by instinct.)
The size of a woman’s hand, it has a hard carapace (hence its name in some parts), and is still harder to remove. It feeds mainly on the host’s blood plasma, and though it does weaken its host, it doesn’t kill him. It returns what it does not consume.
What’s more, it filters its host’s blood, removing unusual chemicals, making the wearer in a way immune to poison (or at least helps to recover faster). The natives use it mostly for their shaman, that have to consume strange things to get into contact with the spirits, etc.
More use was found in the civilised north; where there is politicking and treachery, and poisoning quite common. Some noble men and their advisors, and especially their food tasters have chosen to be hosts for the leech… and there is yet a poison to be found against it. Only a few fast poisons are able to kill the host, if lucky. Thanks to the leech, the good old dagger is coming back into fashion.