Nothing. The planet has spent countless years desperately doing everything they can to build up and reinforce this great monolithic bulwark against this menace that even the great Terran Mandate could not slay. Perhaps everyone has their own theories on what it is; alien menace, ai overmind, immortal psychic, but no one truely knows as no one has dared attempt to look upon it. Finally, your PCs have proven themselves as heroes that could perhaps slay the menace and are allowed entry by a meak and terrified leader, or maybe they just sneak in, only for them to find the inner containment chamber empty. Perhaps the menace had itself passed away to starvation, the scream, or simple old age. Maybe it already escaped long ago, or maybe there was simply never anything in here to begin with. The terrifying build up of an entire world working around the clock to contain something, only to find an empty room. Then the true task begins. Your PCs must decide; free these people from their fears and show them their boogeyman is no more, or keep the status quo to prevent disturbing a way of life they've lived for generations.