Voidrite

Voidrite, more commonly known as Void Stone, is a volatile matter found in the Void. When combusted, voidrite releases a yellow gas which grants a gravity-defying property, and can also be used as fuel, providing exceptional power. The Leviathan Company discovered this while studying samples taken from the Void rift in Alba, and invented a type of flying machine that make use of these properties. Since then, they started to build large number of such airships, thus needed large quantity of voidrite to fuel their fleet. Being exposed to voidrite, however, is dangerous as it dulls the mind and transforms the body. Thus the company resorted to kidnapping construction workers, forced them to work as miners inside the Void hollow. Not having protective gear, the miners were gradually turned into supernatural creatures similar to the Envisioned.

Trivia

 * "Voidrite" is a term used by the Leviathan Company.
 * There are various objects in the franchise that are actually made of voidrite.
 * Billie's Black Shard Arm is one such object, though it is unknown why she is not affected by it.
 * The knife used by Hayward Woodrow, a former Overseer, is made of voidrite. He used it to threaten the owner of Jacko's Emporium in order to acquire some Addermire Solution.
 * It is also possible that the floating rocks inside the Curious Contraptions are voidrite since they are refered to as "pieces of the Void" by the Cult of the Outsider. Adam Christopher half-confirmed this in a tweet.