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).
It is composed of three equilateral triangles, each rotated 0, 40 and 80 degrees. This Nonagram can also be seen on their album, "All Hope Is Gone".
Random BlurbJoey Jordison's new side project, Scar The Martyr debut album out october 1st.
Available to download from the iTunes store