An Incarnation of the Radius (アン・インカーネイション・オブ・ザ・ラディウス, An Inkāneishon Obu za Radiusu?) was a monster developed by Argus to be the boss of the 100th Floor of Aincrad in the official version of Sword Art Online (SAO). However, when Kayaba Akihiko turned SAO into a death game, he intended to take the place of the final boss himself if the floor was reached, leaving the boss unused in the game. Sometime after SAO was cleared, the data for the boss was found by Professor Shigemura Tetsuhiro, who decided to reactivate the monster on the SAO server and use its language engine as the foundation for a Yuna Artificial Intelligence. On April 29, 2026, An Incarnation of the Radius was defeated by a group led by Kirito in order to prevent the professor from conducting a dangerous brain scan on the survivors of Sword Art Online at the New National Stadium. The monster had ten Hit Point bars.
An Incarnation of the Radius is a colossal monster that looks like both a goddess and a beast, as the monster is based on the goddesses mentioned in the Genesis of Aincrad lore. The monster has a white body, red eyes, a feminine face, and several red orbs on its head, which extends above and behind its body to waist length, forming six long, sword-like appendages with a red orb at the bulge on each of them. The boss has two arms, wielding a crimson sword in its right hand, and a spear with a red head and black handle in its left. The monster also wears a lavish, hakama-like piece of clothing from the waist down. The hakama is crimson with black lines at its lower part, dark with red lines at its middle and upper parts, with a golden decoration at its very top.
An Incarnation of the Radius has several ordinary attacks, three special abilities, and a defensive technique. The boss's basic attacks consist of it using its sword, spear, or tentacles to attack at close range, or fire either lasers or missile-like projectiles to deal damage from a distance. The monster's first special attack consists of summoning the Black Great Holy Tree to execute a wide-ranged attack. Its second special attack involves summoning the White Great Holy Tree, the dew drops of which heal the monster; however, it is possible to interrupt this ability by hitting the monster before the dew drops. Its last special attack, called the Great Separation, causes the ground beneath the targeted player to be cut into a disk shape and sent flying into the air, before crumbling mid-air and causing the player to fall. Finally, the monster has a special defensive technique, wherein it summons six magic stones around its body to create a barrier for itself.
Agil, Kirito, Lisbeth, Silica, and Sinon descended to the Ruby Palace after being requested to defeat the boss by Yuna; however, the boss began attacking them before they could even land safely. Although the group managed to break the monster's shield and deal damage to it with a concentrated attack, despite being overwhelmed by the boss's offensive capabilities, An Incarnation of the Radius quickly used its self-healing ability to recover all the lost hit points, before using its black tree attack for a grand counterattack. Just as it seemed like the group would be annihilated by the various attacks the monster employed, Asuna joined the battle, piercing the monster's eyes and earning the group a moment of respite. Before the monster could return to its onslaught, Yui brought reinforcements to the battle from ALfheim Online and Gun Gale Online, including Leafa; Alicia Rue, Eugene, and Sakuya; Jun, Klein, Recon, and Siune; Behemoth and Yamikaze; as well as Dyne and Ginrou. With their combined power, the players managed to weaken the boss enough for Asuna and Kirito to kill it with their final attacks, Mother's Rosario and Starburst Stream respectively.
- An Incarnation of the Radius is a reference to Aincrad's full name, An Incarnating Radius, designating the monster as the incarnation of the floating castle itself.
- According to the author of the series, Kayaba Akihiko likely decided to abandon the use of An Incarnation of the Radius due to believing that no one would have been able to defeat it.
- YUNA's mascot, Ain-chan, is a deformed version of An Incarnation of the Radius.
- ↑ 1.0 1.1 Cordial Chords Part 1
- ↑ Volume 1, Chapter 22
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Ordinal Scale
- ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 4.12 Full Record of SAO the Movie "Behind-the-Scenes" - 8th week Ordinal Scale movie bonus (Translation)
- ↑ 5.0 5.1 5.2 Description based on abec's art for the monster on Full Record of SAO the Movie "Behind-the-Scenes" and its appearance in the movie itself.
- ↑ Volume 5, Chapter 6