Terestrox, the Crystal Marauder Edit
Terestrox is a rock monster that has been mutated by the Earth's core. Born a clear-colored monster, Terestrox was considered an outsider by the monsters for his coloration. Sent to the core, he was mutated to a gargantuan beast as he grew and fed on the special ultra crystals that grow there. Gradually, he grew so big that even the Crystal King grew to fear him.
Terestrox's monstrous size intimidates all monsters and allows him to rule the latter through fear, but the ultra crystals he eats gives him his massive strength.
Terestrox is also able to utilize all the monsters' abilities at once, making him a force to be reckoned with. What's worse, if he can't find an ultra crystal, he will eat a small rock monster!