There aren't many people who will believe that a prosthetic zipper face or gunshot wound to the eye (disgusting as they are) are real, but greyscale from Game of Thrones? That'll really unsettle people for awhile because it totally looks like an actual, honest-to-God infection that someone in 2017 could conceivably have.
Which makes it very effective come Halloween, whether it's for a full-on Princess Shireen, Jorah Mormont, or Stone Man costume, or to just infect a completely different character with greyscale.
Greyscale, which somehow manages to be both disgusting and tragic at the same time, is also called "Prince Garin's Curse," but the reigning queen of greyscale has to be licensed esthetician Lex from Madeyewlook. We've broken down Lex's greyscale tutorial into a small-scale (hah) walkthrough, and linked her video at the end of this Halloween how-to if you want a more in-depth guide.
For this skin disease, you're going to need:
First, mix parts A and B of your skin-friendly silicone. Use three different popsicle sticks so you don't cross contaminate either jar (one stick for each jar, and one stick exclusively for mixing).
Smear the silicone fairly thickly over the area you want to greyscale, then use your chisel to shape irregular scales all over the area. You can then carve crooked lines that extend out from edges of the infected area to finish the look. Leave the silicone to set, reinforcing the lines of the scales as it does (the lines will naturally soften as the silicone sets).
If you're using the alcohol-based paint that Lex recommended, activate the paint with a strong dose of 99% isopropyl alcohol. The more alcohol you use, the lighter the paint will be. Start with a flesh tone slightly darker than your skin, then go over it unevenly with gray. As a final step, outline the edges of the scales with a darker gray (use less alcohol for this step so it will be darker).
That's all there is to greyscale, so feel free to scale it up and infect any or all skin that your Halloween costume will expose. If you want more examples of that, check out Lex's video, where she infects her shoulders, face, and forearm. Be sure to let us know in the comments if you decide to go full-on Stone Man (or Woman), or if anyone at the party actually gets concerned for your health!