Slipknot nonagram1
The Nonagram (9 Point Star) is the symbol that Slipknot has drawn by themselves. They didn't borrow it from any existing image.

The Nonagram, for the band, is a symbol of unity with each other, it's the symbol of 9 people. 9 points - 9 people. Each point represents each band member. The 9 Point Star can be seen on their Vol. 3 Jumpsuits (the Nonagram is located on their right sleeve). And no, the Nonagram is not a satanic symbol. It does not represent Satan or evil, it represents the battle with the fake world, Slipknot's battle. One of the band members that has a Nonagram tattoo is Chris Fehn (#3). Well, as you should all know, Slipknot are all from Des Moines, IA. And the words "Des" and "Moines" have 9 letters when put together. So that tells you something right just there.

It is composed of three equilateral triangles, each rotated 0, 40 and 80 degrees.
Slipknot nonagram3
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Slipknot nonagram2