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Before the fateful Battle of Saturn Portia participated in a rare meeting with the Coven, discussing what to do about Oryx, the Taken King. Portia worried about tipping their hand to Oryx, a fact which Illyn anticipated.
Sometime after the Battle of Saturn the surviving (as it seemed) Techeuns - Illyn, Nascia, Lissyl and Portia herself - would study the power of the Taken and how to control it. During one such session, Illyn inadvertently attracted Riven to their location and presumably caused them all to perish.
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