Sissa, the Spinner of Shadows is one of the Elder Dark..
Story
When the Ashen fell from the Tree of Worlds, Sissna was the first of the Elder Dark to gain supremacy. In her cauldron, she brewed such potent shadows that even Riak and Ukkoto could not withstand.
Legends say she may even be the daughter of Loki.
Tactics
Speculation (SPOILERS)
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.