A Court of Thorns and Roses · Chapter 44
Feyre Solves Amarantha's Riddle
Despite being tortured by Amarantha who breaks her bones and forces her to confront dark memories, Feyre realizes the riddle's answer is 'love' and whispers it before dying.
Outcome
Feyre's sacrifice and realization of love triggers the events that will ultimately break Amarantha's curse over Prythian.