A dark angel has an interesting duality. On the one hand, her angelic nature can remind us of heavenly beauty, but on the other hand the fact that she has fallen from grace means that she's a demon.
The first and most important aspect of the costume are going to be your angel's wings. You can get some from a Halloween store or construct some yourself. Unless you want to be really elaborate, it's probably going to be cheaper and easier to just buy them.
You can get them in a number of price ranges, from simple little plastic ones that you strap around your shoulders, to giant ones with feathers that you can buy at some cosplay stores.
Since you're a fallen angel, just make sure that they're black, otherwise you could be mistaken for a regular angel.
The next thing you're going to want to worry about is your clothes. As with any angel, robes are just fine. You can also try a more modern look and wear regular street clothes if you're an angel who is caught up with the times.
You have a bit more choice here with color. You can wear black, or you might consider wearing a bright, dramatic white to contrast with the color of your wings. You can also try some old fashioned corsets and other period clothing if you're a woman (or, heck, even if you're a man) and you like the Gothic look.
If you're a human-looking angel, you might not need any makeup at all. However, if you want to get in touch with your demonic side, you can buy a demon makeup kit or even get a scary demon mask. Whether or not you're ready to give up paradise, a fallen angel outfit might just be the perfect costume for you. Get creative and fly down from the heavens this Halloween!
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