<1. Oryx killed his sisters in HIS Throne World, and Savathûn later describes the events as leaving her trapped in her own Throne. Him killing his sisters was not final, it was temporary. They would've been trapped in their Thrones forever until Oryx brought them back through bloody rituals involving war and deception. 2. Oryx's Throne World disintegrated upon his death, leaving the Dreadnaught in the material realm. 3. Oryx's death was final, as he was properly killed in his own Throne World. While some of the Hidden Swarm tried to bring Oryx back through heresy or Nightmare essence, the civil war with each other left both sides of the brood broken. This was all Savathûn's doing, as she doesn't want Oryx back. Bringing him back is a HUGE detriment to her plans, arguably the biggest. The only Hive sister that would even want him back is Xivu, who actually loves her family like Oryx did.>