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Biceps Archelon (
True to its name, the Biceps Archelon was an ancient, two-headed turtle that was twenty metres in length and armoured with a thick, dark, gleaming shell that makes its sides virtually invulnerable, leaving only its heads and abdomen unprotected.
The boss has three main attacks: a bite attack from both heads, a watery smack from its side fins, and a charge. A few seconds before the giant turtle begins its massive charges, both heads would point in the same direction, then it would raise its heads high in the air, and its bulk would tear forward.
However, once the boss's Hit Points drop under the ten percent mark, the boss twists up its two heads, front and rear flippers and tail against the sides of the shell to begin spinning and cause a whirlpool. Additionally, in this state, the boss is more resistant to any attacks.
The major parties consisting of members from the Aincrad Liberation Squad and the Dragon Knights Brigade were assigned to fight in the centre, while minor group's - Kirito's and Agil's parties - were assigned to attack the boss's sides. Lind reassured the participants of the battle that the boss's bite and fin attacks were not powerful, thus it was decided to let the ships absorb the damage if necessary. However, as the boss's charge attack was dangerous, the gong equipped on the Dragon Knights Brigade's flagship Leviathan was used to signal the timing of the evasion of the attack.
The battle against Biceps Archelon began at about 15:00 on December 24, 2022. The battle was uneventful until the boss's hit points fell under ten percent. At that moment, the turtle began preparations for its spin attack, while the four ships trained at the turtle's heads continued using Sword Skills in hopes of defeating it before the attack commenced. However, as the boss's defence was increased during its preparations, its Hit Points decreased slowly. Thus, Kirito began rowing madly and used the Tilnel's fire bear ram to strike the turtle's abdomen before it could begin spinning. This attack resulted in the boss's defeat and Kirito receiving the Last Attack bonus.
- Biceps is a Latin word that means "two-headed", while Archelon is a genus of extinct turtles that are the second largest turtles to be documented.
- In the beta test version of Sword Art Online, the 4th Floor boss was a giant tortoise.