Anyone that have play the RPG game Cyberpunk know of rippers, when the nails are removed and knives put in instead. I have found a way to make nails look like knives, very nice for a goth event or for live RPG costuming. Here is what you do:

Let the nail grow so between three and five millimeter show over the tip of the finger. Then begin to file it into a / shape. Very short in one corner and slowly larger all the way around. So that you have a slanting shape. Then wash the hands, make sure they are dry and apply one layer of clear undercoat, if you have something that make the nail stronger apply that to. Then lacquer the nails black. Now comes the fun part. You should get hold of come citadel paints, they are normally used for painting miniature figurines, but they are non toxic and works to paint details on nails. I will however recommend a few layers of regular polish under the paint as the paint stick to hard to the nails, and you want to be able to remove the polish. Also get one standard and one fine detail brush also from citadel.

Add one layer of the color Chainmail over the entire nail. You want the black to slightly show through. The paint a band of about a millimeter whit Mithril Silver color at the top of the nail, along the edge to make the tip of the "blade" look sharp. If you want tiny droplets of the color Red Gore here and there on the nails create a nice effect. Do the Cainmail color whit a basic brush and the band of Mithril Silver and the droplets whit the fine detail brush. Then when dry add a coat or two of clear nail polish as an overcoat, the more shiny the better. And congratulations you have rippers.