The Mark of Darkness - inspired by the The Dark Mark from the Harry Potter Series. The Dark Mark is the symbol of Lord Voldemort and his Death Eaters. It refers both to a magically induced brand that every Death Eater bears on his or her inner left forearm, and to the same symbol conjured in the air by the spell Morsmordre.
House banners representing Gryffindor, Hufflepuff, Ravenclaw, and Slytherin. There's also a Hogwarts one featuring the crest and motto, so you can hang them up in your house's common room, or decorate a Great Hall with all five.