How is it possible for Aes Sedai who have taken the Three Oaths to become damane and use the One Power as a weapon?
They can’t use the One Power as a weapon, not in any conventional sense. This presents some problems for the Seanchan, but then, damane are used for more than just weapons. And from the Seanchan point of view, at worst, an Aes Sedai who has been collared is one less marath’damane running around loose and doing the horrible things that their history tells them such women inevitably do. Remember, Seanchan history records a time under Aes Sedai rule, when no one could go to sleep at night with the certainty they would wake in the morning and Aes Sedai took whatever they wanted and killed anyone who crossed or opposed them. To the Seanchan, just removing these horrors from the board is a win.
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