Faebane is a chemical compound that neutralizes and suppresses High Fae powers. It is not unlike ash.
When Rhysand is taken by Hybern troops, this is used on him. Faebane chains are used to null his powers and contain him.
Faebane is one of the primary weapons used by Hybern against Prythian. It is used on Feyre and the residents of the Spring Court by Dagdan and Brannagh during the beginning of the novel, and succeeds in suppressing their power. Feyre and Lucien are just able to overcome the twins from Hybern before they experience the full effects of the faebane. The compound neutralizes their abilities for a while, and they are forced to proceed out of the Spring Court and through the Autumn Court on foot. Weakened by the faebane, they are unable to fend off Lucien's brothers after being discovered by them in the Autumn Court and are forced to flee. The brothers give chase but are unable to apprehend Lucien and Feyre, who are rescued by Azriel and Cassian on the borders of the Winter Court.
At the meeting of the High Lords in the Dawn Court, Thesan introduces a Fae alchemist named Nuan who developed a antidote for the effects of faebane. Using this, the forces of Prythian are no longer greatly affected by the compound, which is wielded as a chemical weapon by the King of Hybern during most battles.