I'm pretty sure it's the esper Chaos.

Venture back through wood, - If you go back through the Silikawood..
Through city made of stone,
Filled with fiends and spirits, - You can reach the Necrohol of Nabudis.
You'll need your blade to hone. - For a while it can be a good place to train.
Sheltered in a place of sacrifice, - Again, he's in the Necrohol.
Mem'ries are what keep this fiend alive, - This probably has something to do with his history as written in the bestiary.
Protected by four kings of magick, - The four Chaosjets that fight with him are said to be tributes with some traditional elemental fiends from previous FF's.
You must give up your blade to fight. - The attack command is sealed when you fight him.