Katon: Ichi is a Ninjutsu spell in Final Fantasy XI. It deals very small fire damage (usually about 15 damage) and lowers the enemy's resistance to water damage. The Katon: Ichi scroll can be bought from a Ninja guild in Norg for 1561 - 1747 gil.
It is part of the "Elemental Wheel" of Ninjutsu spells. The spells create a six-part wheel of one spell reducing the enemy's resistance to the element of the next as seen below:
The set Elemental Wheel of Ninjutsu consists of the following (Note: these are in casting order, and the casting can begin at any spell in the sequence):