House walkers are strange alien predators which travel the galaxy via spores blown in the solar wind. Once they find a suitable planet they take on an unassuming form in order to lure prey. On the planet Earth their most common form is of large buildings and structures. They are capable of mimicking such a structure down to the most minute detail, including paint jobs, windows, furniture, etc. When an unsuspecting family moves in they're absorbed and the house walker moves on to greener pastures.