I saw this poster recently. The symbol used here is the symbol used in the movie for the deathly hallows. I haven't read the book so I can't say if it's the same or not there. But my point is, does this symbol look familiar to you?
The deathly hallows in the movie are said to be three magical items that together give you eternal life. A magic wand (line), a magic cloak (triangle), and a magic stone (circle). But combined they also unmistakably make the Illuminati's symbol of the all seeing eye in the pyramid.
I'm not trying to spoil the fun for anyone. By all means get dressed up and go to the movie. Go to Harry Potter meetups. Have fun with it. But this is a warning that there is occult symbolism in the movie and we know that the media and popular culture are owned and corrupted by them, therefore you should be aware, that's all.