Rhulk's Vault Might Contain Oryx's Worm
The entire dungeon is heavily Worm themed. If they ever were going to reveal/explain the loose end of Oryx's AWOL worm from Ghosts of the Deep, now is probably the time. The other alternatives concerning the fate of Oryx's worm are as follows:
- Xivu Arath harvested it from her brother. I think this one is the least likely because imo she'd see it as a desecration of her King.
- Savathun carved it from him with her "Scalpel." I don't think this is completely implausible, but I feel like Savathun has been so active in the story over the past couple of years that if this were the case it would've come to light already.
- The worm ate its way out of his body and is just completely unaccounted for (really bland imo).
This leaves the final option. Prior to Witch Queen's story, Rhulk traveled to the depths of Titan and harvested Oryx's worm for his studies & experiments. As awesome as Sundered Doctrine is, its narrative feels very incomplete compared to the other modern (Duality onward) dungeons. We know the dread are being drawn to the dungeon, but we don't know why. Whatever it is, is on the other side of that Vault. So, narratively, the entire payoff for the dungeon is also behind that Vault. This is in contrast to the other dungeons where the secrets fleshed out side lore. For Sundered Doctrine, the main punchline to the narrative appears to be gated behind the weekly quest. Hopefully this means it has some narrative weight - Oryx's worm would fit the bill.