Nothing scarier than the unknown, how about the following:
"The figure is pointing some sort of handweapon at you. It's about the length of their forearm and seemingly made up of free floating crystals, endlessly circling round each and linked by coiling arcs of power. It emits a keening noise that sets your teeth on edge, and there's a sharp taste of iron in your mouth."
The party should be left thinking; I don't know what the Hell that thing is, and I don't want to know while it's pointing at ME!