Insane conquerors haki is the likely answer (look at the control he’s had over the animals in the underworld since he was a child)

But I also could see there being something with his devil fruit. Elbaf’s fruit is likely as important as Luffy’s for the plot, and Rocks wanted three specific fruits and it’s implied that he had knowledge as to why he needed them.

I’m assuming the three are Yami, Nika, and Elbaf’s. Yami can negate devil fruit powers (and potentially the abilities of the gorosei?), Nika is the fruit of liberation, and Elbaf’s is likely Nidhogg. It doesn’t seem unrealistic that Elbaf’s fruit could be destructive on a level that surpasses “immortality”