Puppeteer Slugs
These creatures are very small slugs, usually of a shiny dark green or grey colour. They are scavengers and parasites by nature. They seek out fresh animal or humanoid corpses in their environment. Once found, they burrow themselves into the corpse and start to reproduce. In time, they are able to gain control of their dead host's muscles. (Rumours speak even of ancient Puppeteer Colonies that are able to activate most of their host's abilities, or enough to blend in with others of his kind, and fool even sentient, intelligent creatures.)
They feed on certain tissues, and their waste product is able to halt further natural decomposition of the host, thus they are able to preserve the body.
Once the colony is established (which does not take long; fresher corpses allow for faster infestation) they are able to fully control the corpse and make it appear as though it were living. Depending on the state of the corpse, they may then seek out other corpses of that species to replace damaged limbs or simply create another Puppet. The Puppeteers may “jettison” unwanted limbs at will, and find better replacements. In fact, that is one of their goals as they wander the Underdark; to constantly improve the strength of their hive host.
Without a host, these creatures could be represented in NWN by the smallest ooze model. With a host, they can be represented by any of the humanoid or animal models. (The body may look pale or haggard, but does not look as the rotting remains of an undead creature - neither the host nor the slug is undead).
When in contact with other living beings that are not aware of the true nature of the infested creature, it may try to blend in or remain peaceful. Should the others detect that something is wrong (eyes moving in opposite directions, for example), or if the colony feels threatened or it simply desires the limbs of the other creatures, the host will attack, and newly hatched slugs will pour out of the corpse, trying to escape the hive incase it is destroyed.
So, in NWN, a spawn of this kind could be a humanoid with several young slugs at its feet. The humanoid represented by the half-orc model (because this particular colony found some strong limbs for its host, even if the original host was an average built human) and some small oozes to represent the slugs.
Examine description would vary depending on the appearance of the host. The slugs themselves would be described as “A disgusting mass of slimy, writhing slugs”.
Without a host, the slugs are not that threatening or strong or quick. However, if attacked, they emit a screech that can summon nearby colonies - that have found a host - to come to their aid. So, a special ability could be to summon a nearby host creature (ex: infested rat, rothe, elf). The host's special ability would be infestation of maggots on touch/bite. (The maggots that infest a living creature can not take control of the target until it's dead.)
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Yeah, it's a pretty common idea, and I'm not sure if there are monsters like that in D'n'D already.
EDIT: I forgot to add: Not every colony would wish to blend in, and so would not care about constructing Puppets with body parts of various creatures. There could be cases of a Puppet with the head, torso, legs of an elf and with the arms of a hook horror. Problem with this is that it's prolly not possible to mix models like that, but I know zero about the toolset so I'm hoping.