From Final Fantasy Wiki

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.

Final Fantasy IX Enemy
SoulCage
Statistics Level HP MP 26 9,765 862 Strength Magic Power 37 14 Defense M. Defense 22 10 Evade Magic Evade 4 5 Speed Spirit 24 {{{Spirit}}} EXP AP Gil 0 9 3,800
Elemental affinities Fire Ice Thunder Water - - - - Wind Earth Holy Shadow - - - -
Japanese	ザ･ソウルケージ
Romaji	Za . Sourukēji
Location	Iifa Tree
Class	Undead
Steal	Oak Staff, Magician's Cloak, Brigandine
Item Dropped	Phoenix Pinion, Elixir
Card Dropped	Antlion
Abilities	Shockwave, Fira, Mustard Bomb, L5 Death, Leaf Swirl, Fire Blades
Eat	N/A
Status Immunity	Petrify, Zombie, Stop, Heat, Mini, Venom, Death, Poison, Freeze, Silence, Confuse, Regen, Vanish, Trouble, Berserk, Float, Doom
Other Information	Use curative spells/items for an easy win

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 will not kill him as it will reduce his HP to a mere digit. Either Life or Elixir can kill him permanently.

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, dealing over 600+ damage.

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.