In Queen of Shadows, there is a passage on page 419 where it says "I killed a man for sport when I was and never told anyone where I buried him. I kept his finger bone in a drawer." This sound a lot like Amarantha, because she also kept Jurian's finger bone after she killed him. Amarantha also appears to look like 20 in ACOTAR. My only problem solidifying this theory is that I don't know Amarantha's age.