My theory is that at the climax of the **Void Century** conflict, **Nefertari D. Lily** used the **national treasure of Mary Geoise** along with the Op-Op fruit to spread the poneglyphs around the world and assemble the Red Line, then died after conducting the **Perennial Youth** operation on **Saint Nerona Imu**.   
*(Swipe between panel images as referenced below)*

|*From 1086: THE FIVE ELDERS, Page 13 of 20*|
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To preface, I don't necessarily believe that Imu's immortality is based on the **Perennial Youth** operation. It could easily be based on their abilities that we currently know little-to-nothing about. This entire theory is based on the premise that Imu's survival was a result of the Op-Op fruit, as **Emporio Ivankov** suggested. 

With that assumption out of the way, I believe that during the **Void** **Century** conflict, Lily was outwardly allied with Imu and the Twenty Kings, but secretly a member of the "D" clan and working with Joyboy (parallel to Luffy and Vivi's friendship despite Vivi's membership in the World Government as Alabasta royalty).

|*From 761: THE OP-OP FRUIT, Page 8 of 20*|
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Doflamingo describes how, along with the Op-Op fruit, the national treasure would allow him to ascend to world domination. This implies either that:

1. The immortality granted by the **Perennial Youth** operation would allow someone to make use of the national treasure of Mary Geoise
2. The **Personality Switch** operation would allow him to switch souls with Imu (implies that Imu is the national treasure)
3. Some other ability of the Op-Op fruit used with the national treasure would allow for world domination

Option (1) doesn't really track, as **Perennial Youth** only grants immortality, not invulnerability. Immortality only ensures a long reign after already being a ruler, as in the case of Imu. Doflamingo returned to Mary Geoise in his youth, so it is unlikely that Doflamingo referred to taking control of the World Government at such a young age with only **Perennial Youth**, as he would easily have failed. 

Option (2) also doesn't track because if he regrets not having the Op-Op fruit at that time, that implies that he believes he can **Personality Switch** with Imu, likely the strongest Haki presence in world. Since abilities can be nullified by Haki, there is no chance that a child Doflamingo could do that.

Option (3) seems the most relevant to the national treasure of Mary Geoise. Specifically, I theorize that the national treasure of Mary Geoise is what could be called a "**Room** amplifier" that extends the domain of the Op-Op fruit's **Room** ability to the entire globe. At such a threat level, it makes sense why the World Government would pay ฿5,000,000,000 to acquire it (a value on the likes of Roger, higher than the bounty of any currently living outlaw). 

|*From 1085: THE DEATH OF NEFERTARI COBRA, Page 4 of 20*|
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Nearing the end of the Void Century conflict, Lily realized that with Joyboy's defeat, the current resistance could not beat the Original Twenty Kings. Imu characterizes the liberation of the poneglyphs as **Lily's mistake**, but then considers whether it could have been planned. Once Nefertari Cobra says that Lily's full name was Nefertari D. Lily, Imu immediately lashes out; clearly, it was out of anger of being betrayed by their ally from 800 years ago. 

*From the diagram, steps 1-4 representing the Op-Op Fruit used to form the Red Line*

I theorize that the Original Twenty Kings ordered Lily to use the **Room** amplifier and the **Amputate** ability to slice parts of the previously existing continents, assembling them together at the center of the world in a long, supercontinental strip: the Red Line. 

>Thematically, taking landmass from the previously existing subcontinents and "lowering" their elevation to add to the elevation of the Red Line acts as a direct parallel to the wealth-based inequality existing in the current world; Celestial Dragons take heavenly tributes from already poor nations, economically disadvantaging them while they themselves increasingly accumulate wealth. 

Afterwards, they ordered Lily to dispose of all of the poneglyphs across the worlwide oceans or to drop them into outer space. However, the leeway with this request allowed Lily to intentionally distribute the poneglyphs around the world, which Imu may have considered as the poneglyphs washing up ashore from underwater or entering into the atmosphere from outer space; essentially, attributed it to Lily's mistake in using **Shambles** alongside natural factors. 

After Lily does this, Imu then orders Lily to complete the **Perennial Youth** operation on Saint Nerona Imu, immortalizing them. 

|*From 996: ISLAND OF THE STRONGEST, Page 12 of 20*|
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I think this sets up the end game of the story. While Luffy's journey will culminate in finding the One Piece and learning the truth of the world, I believe that Law's role will directly work to put the world back into "one" piece. Trafalgar D. Water Law's ambition in piracy is to find out the meaning behind the **Will of "D"**, which I suggest is directly tied to the will he inherited from the previous user of the Op-Op Fruit, Nefertari D. Lily; we know that devil fruits represent the passing of inherited will across generations. 

In his quest to discover the meaning behind the Will of "D", he will eventually end up at Mary Geoise with the national treasure and be able to accomplish what Doflamingo couldn't with it, summarily completing Law's character arc that started in Dressrosa. By reversing the damage done by the creation of the Red Line, Law will have accomplished the Will of "D", which I suggest will be related to fostering a greater connection between all people of the world, something impossible with the fragmented island society that the world currenty exists in. 

THEORY: NEFERTARI D. LILY, the OP-OP FRUIT and the NATIONAL TREASURE of MARY GEOISE