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«Ruby Palace» (紅玉宮,Kougyoku-kyū?) was a red palace on the 100th and final Floor of Aincrad in Sword Art Online. If the game had proceeded as planned, this would have been the place where the Clearers who had reached the topmost floor alive would have fought against the final boss, Heathcliff, beating the game if they were victorious.
On April 29, 2026, after discovering the hidden FullDive function in the Augma, Kirito, along with most of his friends, were able to travel to the Ruby Palace in Aincrad, where they fought An Incarnation of the Radius, the original final boss of Aincrad. Although they initially experienced severe difficulty in battling the boss, the arrival of Asuna and a group of reinforcements from ALfheim Online and Gun Gale Online allowed them to defeat it.
In the non-canon Sword Art Online: Infinity Moment story arc, Kirito gets to fight the last boss, Heathcliff, on this floor, though they had fought on the 75th Floor in this arc as well.
Before the release of Sword Art Online, Argus had created «An Incarnation of the Radius» to serve as the boss of the final floor of Aincrad. However, Heathcliff intended to replace the boss with himself in the official service.