His true form is definately a tree - remember THAT forest?

But as for what's inside the Armour, who knows? I don't know what this means, but I felt it was relevant. In the castle where the legendary weapons are sealed, you can fight an enemy called "xdethsoul" or something like that. They look exactly like he does. I think they have something to do with what is in that armour.

But, in the earliest mentions of X-Death(ExDeath) I am pretty sure I heard Galuf refer to him as a mage - a really powerful one. I have anthologies, and not a ROM btw, in case no-one is sure of that. I could have just imagined it, but I doubt it. So maybe it's some sort of magical trickery.