Deep in Grayleaf, you will discover three banshee sisters. Entranced by the song that two of the sisters are singing, you might talk to them. It turns out that one of them has been cursed and cannot speak or sing. If a banshee cannot sing, she will die. It’s now your task to find a cure for the banshee.
To find the banshee sisters, you will need to travel east from where you spawn in Grayleaf. Keep following the road to the east until you find the banshees near a large skull-shaped rock.
After speaking with the banshees and agreeing to help the songless banshee sister, you must find the Tomb of Tuchulkan. Head north of the banshees’ location. The road will fork a couple of times but keep heading north instead of straying from the path. At the end of the path, you will find a building with a dungeon next to it. That is the Tomb of Tuchulkan.
Head inside the tomb and continue straight until you come across a room filled with caskets and two staircases. At the top of the staircases is a group of undead enemies surrounding a book. Defeat the enemies and talk to the book.
After speaking to the book, you will be given the necessary song to break the banshee’s curse. When you return to the banshees, they will tell you a woman’s voice must be sacrificed. You have a few options here. You can either find Lady Darcoil, who is near where you first entered Grayleaf, sacrifice Coridandre’s voice, or sacrifice your own.
Lady Darcoil is more than happy to sacrifice her voice to help the banshee. In doing so, the banshee’s soul and inhabits Lady Darcoil, and the two become one.