Grogg

Grogg has guarded the Power Prism for a millennium. An invader only has to survive chambers full of worm creatures and answer a riddle:

"It cannot be touched, tasted, seen, or heard; it might not exist at all, except that its strength has laid low the greatest titans. All live under its reign, and eventually fleshing, Autobot, and Decepticon will all fall before its just hand.  What is this?"

Well, at least the worm creatures are tough.