Fight your way into the depths of the Circle of Bones, survive the Aria, and defeat the Deathsinger, Ir Airâm. Then, bring her skull to Eris Morn.
"The wicked one who would see the Deathsingers' Choir reborn has been heard in the depths. It is her song—unique in its sorrow—that will complete your mastery of symphonic arcana. Survive the Aria. Survive the depths and her wretched melody, and then collect one final instrument: the Singing Skull of Ir Airâm." —Toland, the Shattered