Wait so are oryx ya siblings alive or not I don't have all the book of sorrows so I don't know
English
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Yes, they were summoned back because after oryx "talks" with Akka, he steals the worm's secrets and gain the power of the deep which is the power of the taken. He showed the greatest display of chaos and war by taking and massacring most of the race they were fighting and using trickery to poison their waters and in doing this he summoned back the other sister.