I have seen the end of my thousand-year life, and it is not now. You cannot stop me. It is futile to even try.
The Soulcage is an entity in Final Fantasy IX. It is the source of the Mist that plagues Gaia, overseeing its dispersal over the Mist Continent from within the Iifa Tree. An undead being who has existed for nearly a thousand years, his visage is a mockery of a tree. He "cannot lie", leading to some revelations in the plot.
Soulcage oversees the physical manifestation of the Iifa Tree and its functions, dispersing Mist over other continents to incense people to war with each other. When Zidane and his allies learn the Soulcage is the cause of the Mist, they kill it and, for a time, stop the dispersal of Mist over the world.
It is later revealed by Garland that the Soulcage the party destroyed was merely an illusion, or the "back door" of the true beast. Soulcage's goal was to halt the flow of souls in Gaia and replace them with the souls of Terra, eventually transforming Gaia into a new Terra when enough of Gaia's spirit energy was turned into Terran energy. The stagnant Gaian souls that could not be reincarnated turned into Mist as a byproduct of this process, and was used to spark war among the people of Gaia to increase the rate at which they died, thereby allowing the Terran souls to replace the Gaian souls faster.
The Soulcage is fought as a boss in the Iifa Tree. He is undead and can use LV5 Death, that kills a party member whose level is divisible by five, and Mustard Bomb, which will cause the Heat status, causing instant death as soon as a party member tries to execute a command. He also uses Shockwave and Leaf Swirl, which hit the party for minor damage, and Fira.
If Fire magic is used on Soulcage, the foliage on his body will catch fire; this will cause him to increase in both aggressiveness and power, using a unique attack Fire Blades, which is an all-Fire attack that deals 600+ damage. This can be reversed by casting Blizzard magic on him.
Being undead, Soulcage is easily defeated with a Life spell, a Phoenix Down, or an Elixir. Phoenix Down will not kill him as it will reduce his HP to a mere digit. Either Life or Elixir can kill him instantly.
If the player chooses to fight him in the regular way, the best strategy is to use Bio with Vivi. If Bio isn't available, use Blizzara or Thundara, as Fire-elemental attacks produce a Fire-elemental counter dealing around 200-300 damage to the entire party. With Zidane, the best is to attack physically and Steal (only if one doesn't have the Oak Staff already). With Dagger, use Ramuh and heal when necessary, and with Eiko use Fenrir (Not Phoenix as it would trigger the Fire-elemental counter) and heal when necessary.
An alternate strategy would be to have Dagger and Eiko both use Cura on Soulcage. As Soulcage is an undead-type, the healing will turn into damage. At level 25 - 30, depending on your gear, Cura will do around 1000 - 1500 damage to Soulcage. For only 10 mp per spell, this is a cheap and VERY effective method of killing Soulcage compared to using Eidolons.
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.