Reverie Arc, or Levely Arc, is characterized by a large and varied cast that comes from all parts of the One Piece world, from royalty to marines to the members of the Revolutionary Army. Some key important characters are Monkey D. Luffy, Crocodile, Koby, Helmeppo, Marshal D. Teach, Sabo, and Stelly, and some other world leaders will be attending Levely.
This arc is mainly caused by the coming-together of world leaders to attend the Levely, a global summit that happens every four years. The major parts of this arc take place in Mary Geoise, the holy land of the World Government and location of Levely.
The plot of the Levely Arc rotates around the global summit of world leaders and several political manipulations around the said summit. The arc opens with many royal families, their representatives, and famous faces, like Vivi from Alabasta and Shirahoshi from Fish-Man Island, all gathering to Mary Geoise. Throughout this gathering of all these leaders, it shows the current condition of the world and the repercussions of the Straw Hat Pirates' actions on most of these kingdoms.
A very large part of the arc revolves around the activities of the Revolutionary Army, mainly Sabo and his group, as they infiltrate Mary Geoise during the Levely. This creates a kind of tension to the proceedings, foreshadowing probably a fight between the revolutionaries and the World Government. Another touched aspect is the shifting balance of power in the world, mentioning how the Shichibukai system was, and the influence some pirate crews are, most notably the Straw Hats, on the rise.
Within this arc, several subplots are either introduced or furthered, including the mysterious vanishing of Cobra, king of Alabasta; the declaration of the system of the Seven Warlords of the Sea's dissolution; and hints of a weapon that may tilt the balance of world power. In addition, it updates many characters from previous arcs who show how recent events have affected them and their perception of the shifting balance in world power.
This may be a very crucial part of the Levely Arc since it serves to lay down future conflicts and storylines. There are new tensions between nations, a turn of allegiances, and hints of major events that will ultimately strike the world of One Piece. The arc works almost more or less as a kind of bridge between the important sagas, filling up certain information and context for the events to come.
The Levely Arc concludes with several major developments that have far-reaching implications for the One Piece world. Although actual resolutions that the Levely has come to with their decisions are not known, it ends with hints that major decisions have been passed to drastically affect the global balance of power. Probably one of the most concrete resolutions would be the disbandment of the Shichibukai system, setting up new conflicts and allegiances for future arcs.
This arc is vital to One Piece both in world-building and plot development. Compared to most of the arcs, it is rather short, and it does not even appear to focus on the Straw Hat Pirates, yet it holds key context in relation to the greater conflicts and political climate of the One Piece world. The Levely Arc adds depth to the series by going beyond the simple adventurous activities of the main characters and providing much wider views on what the actions of each character do globally and on the tangled politics that have been driving their world.
The importance of the arc is that it sets up future events rather than immediate action or character development. It mainly questions what justice truly is, if the World Government should be the one to take control over the seas, and whether pirates are to be in charge of shaping world history. As this arc concludes, it leaves both readers and viewers in anticipation for major conflicts and revelations to come, especially in the Wano Country Arc and beyond.