Okay, so.
Sarda was the most powerful wizard of all time. And he was the Onion Kid. And all this stuff was set into motion when Black Mage tormented said kid, starting with breaking his mind with a glimpse of the dread lattice of black magic.
Maybe exposing his mind to the very fabric of magic at such a young age is what gave Sarda such awesome magical potential in the first place.
This may or may not support this theory.
Thoughts?