Because if it is possible we could help reverse the damage caused by the taken and oryx left behind and who know if the kell of the house of stone was taken we could save the houses that were taken by oryx but I don’t think something like untaken something has ever happened or maybe never will
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The only known exceptions of Taken individuals being freed have been the Techeuns, and, iirc, we don't know the specifications of how they were freed (other than just shooting them a lot).