HP: 5,100

Achelous is one of the few underwater fiends that you can battle (and capture) for the Monster Arena. Because you fight them underwater, only Tidus, Rikku and Wakka will be able to participate in the battle, which limits the number of strategies that you can use to defeat them. That being said, they are very easy to defeat.

Achelous will begin to charge up its “Sonic Wave” attack after you attack it. This attack isn’t particularly strong, but try to defeat it before it gets to use “Sonic Wave” and you can greatly reduce the amount of damage done by the Achelous.

Ability	Description
Sensor	“Watch out when it starts glowing. It releases Sonic Wave after the third pulse.”
Scan	“It starts glowing when hit. After the 3rd pulse, it attacks with a Sonic Wave. Put it to sleep to prevent it from glowing. It takes no damage from water, but is weak against lightning.”

Steal:
Water Gem (x2) (common)
Healing Spring (rare)

Drops:
Power Sphere

Location

They can be in the underwater sections of the Mt. Gagazet Cave exclusively. Find the part of the cave where your party members are required to swim and you will encounter them regularly.