This article is from the Final-Fantasy and other Square Soft Games FAQ, by email@example.com (Nick Zitzmann) with numerous contributions by others.
Mana magic, obtained near the end of the game, temporarily increases the
level of the sword weapon. It's supposedly needed in order to change the
Dragon Buster (level 8 sword) into the Mana Sword (level 9 sword), for
use during the game's final encounter.
You can find level 9 weapon orbs for every weapon in the game - except
for the sword - in random encounters inside the Mana Fortress. (Square
made it so that the Dragon Buster could not easily be turned into the
Mana Sword, since doing so would make the game's last encounter much
There is, however, a cheat which allows you to get a ninth sword orb:
1. When you get to the Ice Forest, go to Neko's and save the game.
2. Reset the SNES & start a new game. After the text in the Mantis Ant's
domain is displayed; press & hold L, R, Select, and Start.
3. The game should reset. Load your previously saved game, and defeat
the Mantis Ant. You'll receive the ninth orb for your sword. The Mana
Sword will soon be yours, hero!
(Note: Your mileage may vary when using this trick.)