Grave leech: a thin boneless mass with a few tendrils it uses to move or grasp, and a single toothy maw on the underside, with no other orifices, sense organs, or internal structures beyond the stomach and intestines. They can be anywhere from cat sized to elephant sized (castes), but all varieties eat carrion, drain blood from living victims, and will raid your food stockpile. Survivors of feedings will often be consumed by larva that were introduced from the creatures' bite, causing massive internal bleeding, pain, and necrosis. If the victim survives, that, the larva will gestate further until it emerges from their abdomen, consumes them, and continues life as an adult grave leech (transformation into grave leech caste roughly size of human) Corpses encountered by a grave leech will also either be consumed outright (reanimation that turns them into a creature that boils away immediately) or used to incubate further larva if the beasts' own appetite is sated (reanimation into small grave leeches. Leech has one interaction to "eat" corpses and several to incubate larva. All have equal cool down. If it encounters several corpses, hopefully it will eat one and raise the others)
- they are naturally crazed, not living, opposed to life, and prone to berserking.
- all individuals are genderless sans the largest castes, one of which is male and the other female, birthing a large number of larva. Smaller castes are more common.
- they have a lifespan of ~60 years and grow throughout that period. Because the game generates individuals around halfway through their lives by default, you can set their average body size for each caste at 30 years, and include slow growth to a larger size over the remaining 30-40 years. Larva could all start out the same size: tiny.
- they live in most damp or cool environments, from taigas to tropical mangrove swamps, and deep underground. Come in groups of 1-10 infrequently and meander about.