October 27: Sethan
You wouldn't expect there to be much to the design of a stand that just takes the form of a shadow, but there are two pretty awesome details about Sethan. For one, it has a big, toothy, bird-like beak when it turns to the side, and for another, it has disturbingly humanlike, non-shadow eyes, complete with surrounding flesh, which manifest on whatever solid surface Sethan is currently cast upon. This single detail makes the stand more interesting than a majority of other shadow-entities I've ever seen, and definitely one of the most unnerving designs in the series.
Sethan's powers are twofold. For one, its appendages can reach out three-dimensionally to attack, usually with the shadow of whatever weapon its user might be holding. More importantly...absolutely anyone caught within this shadow is almost instantly de-aged. It takes only a couple of seconds for an adult to be reduced to a child, and we see one woman de-aged down to a helpless embryo. It's really not even necessary for Sethan to murder anybody with an axe, but its user happens to be a homicidal, violent sadist, so he just does the axe murder part for fun.