Sissa, the Spinner of Shadows is one of the Elder Dark..
Legends say she may even be the daughter of Loki.
It is possible that Sissa was imprisoned by Amiren. This would explain why "Silaren" required relics of the Elder Dark. She would then have taken advantage of the situation when the player killed Amiren to infiltrate Gefn's servants and find the Ashen for herself. It is also possible she anticipated even this and adopted Silaren's form in order to take her place as Amiren's prisoner. This is in line with her deceptive and manipulative nature.