This article is from the Final-Fantasy and other Square Soft Games FAQ, by email@example.com (Nick Zitzmann) with numerous contributions by others.
It's somewhere deep inside the Gateway to the Esper World.
The Atma Weapon appears simple at first, but becomes a deadlier weapon
as your character gains HP. Like Dyrnwyn from Lloyd Alexander's Prydain
Chronicles, the Atma Weapon grows longer and longer with more and more
HP, and delivers a harder blow as well. It's overall one of the game's
most valuable weapons - it can also be the game's most powerful weapon
if it's equipped on a very experienced character.