Part 56 (Reveal of The Great One Piece Pt. 1)
Long ago in the ancient past, the King of the Ancient Kingdom has foreseen the destruction of his Kingdom of the Sun. He is the one who ordered the Kozuki Clan to construct the indestructible book known as Poneglyphs to preserve their knowledge and to spread the book, in many parts, across the seven seas.
The Ds are the direct descendants from the King.
The Prophesized Time to open the Wanokuni's borders has arrived, which is signified by "reading" one of the two Strawhats. To read each Strawhat, it has to be first destroyed:
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The current known people who can decide and read the ancient language are Nicole Robin and the awakened people from the Three Eye Tribe.
The message, decoded by Robin, reads as such:
"Pluton, the Solar Ship of the (Insert Name of Ancient Kingdom here), lies dormant deep in the depth of Mount Fuji. When the thunder in the sky high above roars once again, the seals will break and Pluton the Solar Ship of (Insert Name of Ancient Kingdom here) will be born again the in seven seas."
The other Strawhat in Im's control is...
Part 57 (Reveal of The Great One Piece Pt. 2)
The message decoded by Robin above, in Part 56, belongs to Luffy's Strawhat. For the message to appear, the Strawhat has to be first destroyed and then the pattern of the knots along each straw pieces will then finally be revealed.
The Strawhat Crew returns back to Wanokuni. Under Luffy's direction, Momonosuke ordered Zunisha to open the country's borders.
The destruction of the borders resulted in a thunderous roar that resonated across the seas and towards the sky above, and the walls surrounding Wanokuni finally falls.
A large part of the New Wanokuni falls into the sea as well, and the Old Wanokuni appears in all its splendid glory. Pluton emerges from the deep depth onto the sea after a long 800 years of sleep.
The other Strawhat in Im's control is "big" not because the previous owner is a giant. The previous owner is Joyboy as well, who is of similar size to Luffy. Joyboy's Strawhat is bigger than Luffy's Strawhat because it contains a greater amount of knowledge. The act of "reading" it can greatly change the fate of the world depending on the owner of this knowledge.
Blackbeard Crew appears at Wanokuni. World Government appears at Wanokuni. With the help of Luffy's allies, they have been fending off the attacks from these enemies. Using the "Coup de Burst" powered by the Solar Energy on Pluton, the Strawhat Crew, together with Thousand Sunny, escaped.