What is creepier and darker than death itself? A truly scary Halloween costume should not only disturb people visually, but it should ideally touch a part of their deeper fears and leave them feeling unsettled.
This is a Grim Reaper that is draped in the classic cloak and hovers around with the stereotypical scythe. The key to this style of reaper is his lack of obvious identity; this is what really creeps people out.
He appears to have no face, doesn't speak, and is hardly expressive with his body movements, instead gliding around the world of the living, looking for his next victim.
For this costume, you can find a jet black robe that will cover your whole body and feet.
You can find these kinds of costumes at party stores, but the material is often kind of cheap, so you may want to hit up a thrift store and see if they have anything gloomy and black that you can make into your reaper robes. Heck, even your old graduation outfit might work if the color is dark enough. Add a scythe (that is fake or unsharpened) and you're ready to stalk the mortals.
This is probably the simplest kind of reaper costume, and actually is a very simple and easy Halloween costume in general. Think of it as a step above wearing a white sheet and calling yourself a ghost. If you're looking for something that will make a bit more of an impression, you might want to try one of the more elaborate reaper styles below.
This is when things get scarier. We don't really know for sure what the reaper's face looks like, but usually the image in most of our minds is of a skeletal figure. Like the classic style reaper without a face, you can achieve this look fairly easily by throwing on a robe that covers your whole body and includes a hood.
Instead of leaving things up to the imagination, however, you'll wear a skull mask so that when trick-or-treaters peer into your ghostly face, they will see something eerie staring back at them.
Alternatively, you can also paint your face in a skull pattern or use latex prosthetics, but a mask that covers your whole head and hides your humanity completely is probably your best bet.
If you want to get even more elaborate, you can try to imitate a specific depiction of the reaper from any number of dark movies. This can get more expensive, though, but if you really want to make an impression and freak people out, there's nothing like taking a cue from Hollywood.
Some choice movie reapers may be hard to achieve, like the Angel of Death from the Hellboy series, for example, but you might be able to go with some classics like Death in The Seventh Seal. If you want to be clever and subtle, you can just dress as the main character from Meet Joe Black, but that only counts as a Grim Reaper costume on a technicality.