Omniterium

Omniterium is a black, rock like substance that comes from the void with mystical properties. The rock is considered precious as it is the only known material to leave the Void, and is utterly unaffected by it's pull. Because of this, it is believed to be the stuff of the Chaos Dimension, which lies just beyond the Void. Omniterium is also the only material that can aid in hiding life signs from The Collectors, the phantom like titans that conduct The Reaping. The material also does a good job in manipulating light which aids in cloaking and invisibility. Ancient races have used this to their advantage in preventative measures against The Reaping.

Illegal Nature
Omniterium is a considerably illegal substance, by order of the Order of The Gods, themselves. This was outlawed by Drem, who told his Order prophets that the material is unholy and interferes with the light of The Gods. Of course, the real reason is that the metal makes it impossible for the Collectors to detect life within a container made entirely of the substance. This would omit certain life from The Reaping, allowing them to live past and record it.