Giygas is unable to be explained
Okay so this alien baby is raised by two people but one day it's real parents come and forget up the adopted parents and he's like all broody and stuff and he now loses faith in everything but the first heroes sing to him reminding him that he still loves his adoptive parents but after losing it because pokey is constantly messing around with him and he turns evil and then becomes evil itself and "cannot think rationally anymore" so you have to travel back in time as the next heroes and defeat him while he's in his weakest state, birth of new form.
after defeating him, pokey retreats to another dimension where he wreaks havoc on an island containing the last surviving humans and the last heroes have to defeat him before the kills off the entire race.