Wyrdstone is a smooth, obsidian stony material. A strong magical conduit, it can cause headaches in those with magic in their blood. Jewelry made from it can lodge Valg; donning such an item allows the Valg to possess the wearer. The size of the jewelry seems to determine what rank of Valg can inhabit it. For example, a Wyrdstone ring can hold a lesser Valg, but only a collar is capable of containing a Valg prince. The Wyrdkeys themselves are also hewn from Wyrdstone because of its magical conductivity.
The exact process by which Wyrdstone is created is unknown; it seems to exist naturally only in the form of the Wyrdgates, from which the Wyrdkeys were taken. All that is known is that the Wyrdkeys, and even only a single Wyrdkey, can be used to craft more.
Items created from Wyrdstone include:
These seemingly harmless garments were created by Erawan, and used to house Valg soldiers of varying power. Through wearing these garments, humans became possessed by the demon within them.
The rings were simple black items that allowed the possession of humans by lower-ranked Valg demons, whereas the collars, described as black torques, contained Valg Princes of considerable strength and power. Removal of the Wyrdstone items was enough to weaken the demon within a person and banish them, however possession by Valg Princes and Princesses required additional banishment by a healer.
These towers were created by the King of Adarlan using the Wyrdkey in his possession. A total of three were constructed: one in Rifthold, Noll and Amaroth, all forming a triangle that covered the entire continent of Erilea. The towers were constructed during Adarlan's conquest of Erilea, and remained dormant until the fall of Terrasen, at which point the King activated a powerful spell that canceled all magic on the continent, using the towers as nexus points.
The Rifthold tower was destroyed ten years later by rebels, aided by Aelin Galathynius and her friends. The spell collapsed as a result, and magic was freed.  It is unknown what befell the remaining towers in Noll and Amaroth.
The Witch Towers were a more recent creation by Erawan and his armies. Each rose a hundred feet high, the entire structure built into a platform. Window slits were interspersed throughout every level, portals through which to angle the power of the Witch Mirrors which were capable of being adjusted to any direction, any focus. The purpose of the towers was to amplify the destructive force of the Yielding.
They were all destroyed in the third Valg wars.
A sub-creation of the rings and collars, the hearts were a more direct method of possession that were implanted into humans in place of their hearts. Wyrdstone hearts were a mixture of Wyrdstone and iron, and were used in special cases to create stronger soldiers and assassins without using a Prince.