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".
In 2014, during the pre-release of the album .5: The Gray Chapter, Slipknot released a brand new nonagram (pictured right).