Quote Originally Posted by ThroneofDravaris
This belongs in the FFX forum, but whatever...

Either he needed to transform into that bug thing (Majora's Mask anyone?) in order to Summon the Dream of the Fayth, or he became like that because there was no one left who remembers what he truly looks like
The "bug thing" was essentially the physical manifestation of Yu Yevon's soul. He himself would have been the "foundation" for the original Sin, as the tradition of the Final Summoning had not yet been enacted. Following the original "defeat" of Sin, Yunalesca's Final Aeon would have been possessed by Yu Yevon, that "thing."