Interestingly, Big Mom in Chapter 999 alludes to the God Valley Incident. When she first pitched teaming up to Kaido she said he owes her a lifelong debt and we finally learn this chapter the deed for which Kaido owes Big Mom.
It seems that Big Mom gave Kaido his Mythical Zoan Devil Fruit on God Valley the day that the Rocks Pirates fell apart. – Which seems significant. Why that specific day?
Sengoku says that Rocks D. Xebec was killed by Roger and Garp – and perhaps this isn’t a presumed dead situation. Personally, I do believe Xebec himself is dead, though his will likely lives on in Marshall D. Teach.
We know that the ‘soul’ of the Devil Fruit transfers and regrows following the death of the person by whom it was possessed. What if Big Mom came to possess the Uo Uo no Mi, Model: Seiryu on God Valley because it once belonged to Rocks D. Xebec, who died during the incident?
I know many people have speculated that Teach could eventually come to possess Kaido’s Devil Fruit, should Kaido achieve that which he most desires in a glorious death – and perhaps now there is a thematic connection if Kaido’s devil fruit once belonged to Rocks D. Xebec. I’ve never been particularly partial to the idea of Teach taking Kaido’s Devil Fruit before, but at least this connection could make the idea a little more interesting.
It’s still entirely possible Big Mom came to possess the devil fruit from someone else – perhaps the Celestial Dragons targeted on God Valley.
Ironically, Big Mom says Kaido owes her a lifelong debt, which likely implies that his possessing the Uo Uo no Mi, Model: Seiryu is somehow responsible for his survival. As I said, I believe Kaido’s dragon form affords him good fortune and is responsible for his winning streak, yet his inability to be killed has also deprived him of that which he desires – a glorious warrior’s death. And so, Big Mom granted him both his greatest gift and his greatest curse.
*Theory by StrawHatJedi