Soulcage

The SoulCage is the source of the Mist which plagues Gaia in Final Fantasy IX.

SoulCage oversees the physical manifestation of the Iifa Tree and its functions, utilizing the by-product of mist to incense the will to fight in civilization. An undead being who has existed for nearly a thousand years, his visage is a cruel mockery of a tree. Strangely, he "cannot lie," leading to some revelations in the plot.

SoulCage, right before being fought, states that he has "seen the end of my thousand-year-life, and it is not now. You cannot defeat me", and after the battle, he supposedly dies. It is later revealed by Garland that it was merely an illusion, or the "back door" of the true beast. SoulCage's goal was to halt the flow of souls in Gaia, making resurrection and birth impossible, and ending all life on Gaia. This was so that Gaia could become the new Terra, by absorbing Gaia's spirit energy. However, Trance Kuja destroys Terra, leaving all of the souls assimilated into the Iifa Tree dormant. After the party defeats Necron, the Iifa tree begins to wilt.

Battle
The SoulCage is fought as a boss in the Iifa Tree. Being undead, he is easily defeated with a Life spell, a Phoenix Down, or an Elixir. However, Phoenix Down can be unreliable, as it can miss. Life and Elixir are most effective.

If you cast Fire Magic on SoulCage, the foliage on his body will catch fire and he will increase in both aggressiveness and power, using a unique attack called Mustard Bomb, which will cause Heat status, causing instant death as soon as a party member tries to execute a command (although this attack can be used before fire magic is used on him). This can be reversed by casting Blizzard magic on it. He will also start to use fire blades, which is an all fire attack which can be devastating to your party.

Etymology
In the Spanish version of Final Fantasy IX, SoulCage was translated as 'Onodrim'. In J. R. R. Tolkien's Middle-earth universe, Onodrim is the name given by the Elves to the living trees that humanity called the Ents.

Onodrim