In the page where Cobra mentions Nefertari Lili’s full name, we can see that an arrow shot through the room, and then through Cobra.
Where does the arrow come from?
We can see that the arrow going through the side of the room and ultimately impales Cobra from behind.
But who shoots it?
So here is the thing, all of the five Elders are shown to still be in human form, and on this chapter (and last one) it is shown that the only ones in the room are the Elders, Cobra, and Imu. Since we’ve ruled out the Elders then that must mean that it was shot by Imu. But what does this mean?
How can Imu shoot an arrow that travels the whole room and impales Cobra through his back?
Well, here is the thing. I know it looks like an arrow, but do you know as what else does it look like? A tail. But not any tail. A DEVIL’S TAIL.
The Devil’s tail
I know what you are thinking: in what way does it looks like a Devil’s tail? Well, here’s the thing. If you search Devil’s Tail on google images, you can see that the lines of the bits that connect the arrowhead to the tail form what is an acute angle (an angle of less than 90°).
And the thing is that, on the arrow that shoots through Cobra, the bits that connect the arrowhead to the tail form what is an acute angle.
What does all of this means?
An important fact is that Imu’s name backwards is Umi (sea in Japanese), which is the weakness of all the devil fruits.
And what ability does the tail shows?
The ability to impale and cause damage to Sabo who has a Logia devil fruit! This shouldn’t be possible because everything goes through Logia (except exceptions like haki or Blackbeard’s devil fruit).
Well, here is my theory. Imu’s Devil fruit is the “Devil’s Fruit”, which nullifies all devil fruit abilities.
Theory by One2ruleall (https://www.reddit.com/user/One2ruleall)