In the Books of Sorrow, Oryx is shown to show sorrow for the races he'd killed, and even initially tried to broker peace, only to be killed by Savathun. From what I could read, Savathun is the most merciless, she killed tons of races and never showed any sign of remorse. We have also seen, that through the Books of Sorrow, that Oryx is never separate from his sisters. This means that Savathun and the last remaining sister is either in our solar system currently, or is on their way. So perhaps we'll see them in Destiny 2?
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Honestly, I'm not too concerned about Xivu Arath. They're a god of war and brutality, one who seems to be the least intelligent of the triad. She's more likely to simply attempt to drown us in a bloody tide than anything else. The one that TERRIFIES me is Savathun. She's a trickster, manipulator, and web-weaver. She seems to play the long game, and rely on subterfuge and treachery. For all we know, she might already have an "in" to the Tower or the Reef, or have plans for a Manchurian agent ploy.