Originally thought to have been a gesture of love from Athril to Queen Maeve of the Fae, it's true purpose has now been revealed to offer immunity from the Valg. As, shown in Kingdom of Ash, when Aelin Galathynius put the ring on Maeve, killing her.
Heir of Fire
At Doranelle, Aelin bargains with Maeve for her to release Rowan Whitethorn from his blood oath in exchange for the ring, not knowing the true nature of it. Aelin threatens to burn the people of Doranelle saying that Maeve's city may be made of stone but her people aren't.
Queen of Shadows
Lorcan steals the ring and brings it to Erilea. He trades it to Aelin in exchange for a false Wyrdkey and reveals that it can protect against Valg.
- Ironically, it was used to kill the beloved of its former owner.