Arcs, or Major Disputes, are Disputes between 2 or more City-states of Valoran that are taken to the Institute of War and the League of Legends to be settled through months of matches between the actively participating factions.
This Arc Started when a mysterious meteorite landed in international territory creating a pool of mysterious silvery substance. 3 Factions initially began the Dispute, Demacia, Ionia, Noxus, and Freljord joined in later on and eventually won control of the "Mirrorwater" which was divided among the sub factions of the Freljord. Ashe and her Avarosan built a fortress out of it, Sejuani and her tribes made powerful weapons, while Trundle gave his share (Secretly) to Lissandra and she currently has it kept in secret in hopes of using it eventually to bring back the Watchers.
One of the side effects of the appearance of the Mirrorwater was the creation of a strange island made of a metallic silvery blue material similar to the Mirrorwater, On the Island was a powerful nature spirit Dubbed "Ceruleana" who could exercise control of the Seas. 6 Factions started off, with 2 factions withdrawing part way through. Demacia eventually won control of the Island and purified the island, bringing Ceruleana under their control as a spirit of Order, greatly enhancing their naval strength, giving Demacia the greatest Navy in Valoran.
As Ionia predicted during the Ceruleana dispute, the purifying of Ceruleana to the side of Order caused a massive disruption in the Cosmic Balance. This lead to a void rift opening up and a massive swarm of voidlings to sweep over Valoran. Many city-states fell before the might of the Massive Void swarm, assissted by Aatrox, Malzahar, and Shaco. All other factions eventually banded together and held off the Void invasion at the gates of Noxus. The Void was officially recognized as the Faction of Icathia, and the Void champions that were under the control of the Institute were freed. The Valoranian Capital of the Void territory is the ruined city of Icathia.
Following the devastation of Noxus lands due to the Void invasion, Noxus once again attempted to invade the southern islands of Ionia as a place to give shelter to the many displaced refugees. Ionia having had enough of Noxus's attempts to take control of it's lands fought against the action. This eventually escalated into all out war between Noxus and Ionia. Ionia once again repelled the Noxian invasion and Noxus was absolutely crushed. Due to events during this dispute, Riven had begun a reformation, but when Swain ordered Chemical bombings on Ionia when it was clear that Ionia was winning, Riven turned her back entirely on Noxus, though many reformists live in secret in Noxus.
As a result of the void invasion, much of the sandstone in Shurima deserts were corrupted with Void energy. Mixing with ancient shuriman magic, this dark sand was a source of powerful arcane energy, providing an alternate fuel for hextech technology. Zaun, Piltover, Demacia and Bandle City rushed to Shurima for control of the Dark Sand to advance their technological advancements. Though Zaun ultimate won control of a large portion of the Dark Sand, Piltover and Bandle City were able to achieve huge leaps and bounds. Bandle City managed to find out how to refuel their ancient mothership with the Dark Sand and were able to travel to the moon and contstruct a lunar base. Zaun became a superpower city-state rivaling the power of Demacia and Noxus. During the arc, a Minor Reformist rebellion occured in Noxus, but was quickly crushed with the help of Zaunite reinforcements. Riven and many of the Noxian outcasts had moved to Piltover and have become fully integrated with Piltover after they had been healed due to medical advances made by Piltover into void contamination research and curing void contamination sickness.
As a result of the vantage point from Bandle City's new lunar base, they spotted a new continent upon the face of Runeterra. This continent was surrounded by a shimmering magical shroud shielding it from view from the ocean below, and it was in the midst of the Conquerer's Sea to the West of Valoran meaning that the area was filled with everlasting storms and few ever survived expeditions there. Thanks to the technological advances from the Hextech Revolution, airships were able to travel to the new continent. Over the coming months, the Freljord, Ionia, the Shadow Isles, and Bilgewater would all work to find out more about Nyroth and restore the devastated continent. Ionia ultimately came out as the overall victor of the Arc, but Bilgewater gained many rewards and opportunities that have led many to say that Bilgewater was the true victor of the dispute.