According to the Book of Sorrows, Oryx was able to take because of the Tablets fo Ruin. Once acquiring these tablets, Auryx became Oryx, The Taken King.
Auryx the First Navigator set upon his god with his sword and his words, and cut Akka to pieces, and took from those pieces the secret of calling upon the Deep. He wrote this secret on a set of tablets, which he called the Tablets of Ruin. And he wore them about his waist.
Then Auryx said, “Now I may speak to the Deep, the beautiful final shape. I will be King of Shapes. I will learn all the secrets of our destiny.”
His speech to the Deep is not recorded here. But it is known that he returned, and he said, now I am Oryx, the Taken King. And I have the power to take life and make it my own.
I also doubt that Xivu Arath would come to kill us before Savathun because Xivu Arath likely knows to fear the guardians because if we were able to kill Oryx, we would likely be able to kill Xivu too. Also, Xivu doesn’t have an army of Taken at (her?) disposal. Savatun would likely come after us because she is Smarter than Oryx, and she can now take, as we know from the Lake of Shadows strike. She has also been sending all sorts of scouts, per se. Including things like Savathuns Song, the boss in an Io adventure, Iraz, Eye of Savathun, and other bosses regarding Savathun. She is scouting us out and wants our system for herself. Where Oryx failed, she is confident she will succeed.
Ghost: Zavala, the Taken leader is down for the count. Surprise of the day: he was Red Legion.
Zavala: Impossible… Oryx is long dead. There’s no one left to take.
Ikora was telling me about a Hive tome called the Book of Sorrows. That Oryx has sisters.
Zavala: You both dealt with the Taken on Io. I’ll consult her about this. In the meantime, excellent work. All of you.
Source: Lake of Shadows Strike
Guardian defeats Iraz, Eye of Savathun.
Ghost: Hey, team? The screaming thing from the abyss is dead.
Ikora: That creature was the servant of some higher power. I fear something has claimed Oryx’s crown.
Source: Adventure- Unexpected Guests Transcript
Please fact check me, I’d rather not be spreading false info.