Mordecai explains in The Gate of the Feral Gods, "Sometimes the soul of a creature can get... split... into two halves. When a body is split, the two halves will always try to reunite, like magnets coming together. They have to do it in a proper vessel. But if something goes wrong during the reunification, when you end up with is a withering spirit. They're not alive. Not dead. In fact, it's kind of hard for them to die now, but they're mostly harmless. The vessel has to be similar to their original body, and if it's not, this happens. They haunt the object, but they have very little power."
Samantha is a withering spirit.