A magical layer of slick, black ice covers the floor. This ice blends in to the floor if the floor is dark or takes on the appearance of a shadow. A simple perception check reveals that this "shadow" appears to be missing an opaque source (in other words, there's nothing in immediate view casting the shadow).
A Perception check DC 15 may identify the black ice as a magical ice slick. The ice is unaffected by the local weather or temperature and can only be countered by magical fire (or dampened or dispersed by magical salt or something similar).
Stepping onto it requires a Reflex or Dexterity save, or fall prone and take 1d4 cold damage.
This trap is best used as foreshadowing of an evil ice-based magic user lying in wait.