Wyrdkeys are used to open Wyrdgates.
The Wyrdkeys were first made by the Valg kings to control the Wyrdgates. There are three Wyrdkeys in total, one for each Valg king. After defeating the Valg, Brannon Galathynius took possession of the Wyrdkeys. However, the Fae Queen Maeve stole them from him, killing Brannon's friend Athril in the process. With the help of Mala, the Sun Goddess, Brannon successfully took back the Wyrdkeys and sailed to another continent to get them away from Maeve. He hid them leaving a lengthy riddle/poem in the tomb of his daughter Elena Havilliard in hope that someone good might find them and restore them to their rightful home, in the Wyrdgates.
The first Wyrdkey was located in the crown of Elena's stone-carved sarcophagus in her tomb. It was found by Duke Perrington who used it to open the tomb of the Valg king in Morath, who then siezed Perrington's body.
The second Wyrdkey was implanted in Kaltain Rompier's arm as an experiment. Her magically-gifted bloodline made her a subject of interest. Kaltain possessed fire magic before it disappeared, but once the Wyrdkey was implanted on her, those abilities shifted into shadowfire. Kaltain later carved it out from under her skin and gave it to Elide Lochan, Lady Marion Lochan's daughter, to give to Celaena Sardothien (Aelin Ashryver Galathynius) in favor of the kindness she showed Kaltain in the palace dungeons at the beginning of Crown of Midnight.
The third Wyrdkey, the Amulet of Orynth, is now in Aelin's possession. Originally, the First King of Terrasen forged the Amulet and hid the Wyrdkey inside it, deeming there no being a safest place to hide it other than a royal's neck. The Amulet was passed down Brannon Galathynius's line, allowing Terrasen to prosper within its borders without assuming imperialist intentioms. As such, Terrasen stayed within its borders as generations of noble rulers unknowingly possessed the Wyrdkey. The night before Evalin Ashryver's and Rhoe Galathynius' assassination, Aelin received the Amulet as a gift from her mother. She assumed she had lost it on the Florine River's current; however, as she later found out, Arobynn Hamel had snatched it from her neck while she was unconscious and had hidden it ever since. The Amulet of Orynth's change of ownership also explains Terrasen's doom (the fall of "the most powerful court in the world") and Arobynn's rise towards becoming King of Assassins.
The Wyrdkeys have been destroyed.