Call of Duty may be one of the best video game franchises ever, but I doubt you'll see very many people dressed up as COD characters this Halloween. As awesome as the games are, there just aren't any memorable characters when it comes to looks—besides one.
Simon Riley, otherwise known as Ghost.
Ghost was first introduced into the Call of Duty series in Modern Warfare 2, and has appeared in multiple main and sub-series games, including the upcoming Call of Duty: Ghosts, where he's featured merely as a customization option, though he's not the only one wearing a skull mask this time.
The mask (or more correctly, balaclava) that Ghost wears has an image of a skull on it, which makes it pretty much the only Halloween-worthy costume from Call of Duty. Hell, it'd also make a great winter head covering to keep you all nice and toasty. Or just a badass wartime mask.
Earlier this year, a French soldier caused controversy for wearing such mask because of the violent nature of the video game series, despite the fact that real-life U.S. troops donned it first—and even Christian Bale.
Overall, the costume is fairly easy, and would probably even work well without a military uniform or M4 carbine rifle. There are a few YouTube videos online of people making Ghost skull masks, but my favorite is from Instructables user Luis Linares, who uses the rougher skull mask look from the new Call of Duty: Ghosts as inspiration—not Simon Riley's version.
For the balaclava, he just used the sleeve from a long sleeve black shirt, and white fabric paint. Super simple, just like it should be. The rest is up to you.
Make sure to check out his full guide for more pictures if you need help.