An Armachthys is a large, black with a large gray spot fish that has a pair of large fins in its front, three more pairs of fins on its bottom side, with the last pair being larger than the first two, and two more large fins on its top side. The monster's head is covered in tough bone armour with a horn sticking out of its top. There is a small opening in the armour at the top of its head which may be its weak point. The monster has two bright red eyes and sharp teeth with two large fangs at the center of its jaw.
Due to the monster’s armoured head, it is practically impossible to harm it from the front. The monster usually attacks by charging at its opponent and ramming them with its armoured head, but it may also start swimming fast in circles to form a whirlpool that sucks in players and leaves them off-balance.
An Armachthys emerged from a trap in the Undersea Temple after Kirito and Klein accidentally fell into it and barely managed to swim out of it. Kirito tried to attract its attention while Klein, Agil, Silica and Lisbeth attacked it from the sides while it was busy and Asuna and Leafa supported the party with magic. However, in the midst of the battle, Leafa decided to join in on the melee, but this caused the Armarchthys to become aggravated at her and respond with its whirlpool ability that sucked in Leafa and made her almost fall into the trap. While the monster was still maintaining its whirlwind, Kirito used its force to reach the ceiling and attack the monster from the top, killing it by hitting the chink in its armour.