Blood Drain is the bloated leech's special attack. This special heals stat damage, Poison, and Disease but sacrifices some Life Points.
The leech's blood drain scroll can be used to quickly restore statistics when eating Spicy Stew.
Medieval European medicine was based on the concept of Humorism, in which illnesses resulted from an imbalance in one or more of four body fluids: blood, phlegm, black bile, and yellow bile. The heat and flush associated with fever were believed to be a symptom of excess blood in the body. Leeches were one method used to draw out blood in an effort to restore health by restoring the fluid balance. (This is also why medieval sources refer to the practice of medicine as "leechcraft".)
The bloated leech familiar has two stalk eyes. An actual leech has no eyes, let alone eyes on stalks.
The bloated leech familiar more resembles a certain species of red slug (minus the teeth).