Wyrdmarks are magical runes used to perform magic and cast spells. Although their powerful magic is mainly used for evil, they are a neutral magic which can also be used for good. Known uses for Wyrdmarks include healing, sacrificing, opening/closing portals, creating wards/barriers, summoning, controlling others (maybe?), ensnaring and protection. 

To be used, Wyrdmarks must be drawn on a surface (skin, ground...) or chanted (like Nehemia at end of ToG). Their effect can be altered or ended by changing or erasing the marks or until the spell runs out (like when an effect happens or a portal collapses). When its power is activated, wyrdmarks may glow a bright turquoise/blue. 

Even when the King of Adarlan used the Wyrdkeys to block magic from the world, as Wyrd magic was more powerful and archaic, its magic wasn't diminished. Their magic wasn't used in defense from the King's colonisation as there are little known users or even people who know of its existence. 

Known users of their magic include: Nehemia (and her family), Celaena (limited use) , Elena, Cain and the King of Adarlan (at least knows of them). 

Throne of Glass

Within Throne of Glass, Wyrdmarks are seen carved into the ground around the black, foreboding clock tower at the start where Celaena first learns of them and starts researching them. They are further discovered around the bodies of the champions killed by a beast. It is later discovered that Cain used a set of Wyrdmarks to open portals to another dimension to summon a Ridderak, a terrible ugly beast with venom as poisonous as a king cobra, and mark his sacrifices to it to gain their strength for the tournament. When Celaena was bitten by the Ridderak, Nehemia reveals her ability to use wyrd magic and saves Celaena's life and explains how she has been closing Cain's portals and banishing the Ridderak. The King's dark rings may be a form of Wyrd magic but this is unknown as they could be his own magic he possesses like Dorian's. In the final duel, wyrd magic is used for and against Celaena. Throughout the battle, Cain causing her torture by beasts of other dimensions and Nehemia (summoning Elena) and Elena purging Celaena of the Bloodbane, causing her to see the other dimension. Celaena's forehead glows blue showing, a wyrdmark meaning "nameless" or "anonymous". In which it had purged the poison and her strength renewed.

Crown of Midnight

Celaena further learns of Wyrdmarks and their power and the Wyrdkeys blocking magic from the land. Wyrdmarks are used with Dorian's help to seal the horrible humanoid abomination made by the King in the library's catacombs and to open and close a portal made by Celaena to see Nehemia.

Heir of Fire

There is believed to be Wyrdmagic imbued in the collars on the King's creatures and Dorian to exibit control like the dark rings. 

Empire of Storms

There are Wyrdmarks on the altar in the sunken temple in the Stone Marshes, where Aelin was sent by Diana "to find the lock", and on the witch mirror she found there. Aelin and Manon saw that there Wyrdmarks all across the obsidian sarcophagus into which Elena had sealed Erowan a thousand years ago, as well as on the Eye of Elena itself.