In the SNES game Secret of Mana, all the weapons seem to have 8 orbs, and thus 9 variations, except the Mana Sword. It doesn't seem possible to forge the Mana Sword more than 7 times, to get it to level 9. Is there a way to forge the 8th level sword, the Dragon Buster? And if so, what is the 9th sword?
In the standard gameplay, there is no 9th orb for the sword. I've placed the reason for this in the following spoiler block.
There is, however, a glitch that can let you use it normally. You can find this information in this other answer. ♪
The 9th version of the Sword is the Mana Sword. The only way to get this during "normal" gameplay is via Dryad's final spell, "Mana Magic". When both the Girl and the Sprite cast "Mana Magic" on the Boy, the Dragon Buster becomes the Mana Sword.
There is, however, a way built in to the game to forge the Dragon Buster one final time to obtain the Mana Sword before you get Dryad's magic:
This process can be repeated to teleport your characters to any part of the game, not just the Mantis Ant. However, most locations will not still have the bosses, only the Mantis Ant seems to reappear and give you an Orb. Also, while you can repeat the process to obtain more than 9 orbs, if you forge the Sword past level 9, the game will get "confused" and the sword will change to a different weapon, like the Gloves.
I added, as a comment, to Josh's answer that the glitch won't work on the iPhone version of the game. In fact, the island is different (it's a different shape) and you can no longer land on it:
If you're brave enough to jailbreak your iPhone you can hex edit the save file to change one tiny byte to give you 9 Sword Orbs so that Watts will reforge it into the Mana Sword.
To do this, you will need:
Take great care whilst editing the hex file. Any values must be overwritten, never inserted. Changing the offsets of other values will corrupt the file. It's best to make sure the file size never changes.