Our brave hero has outsmarted the guards, sneaked into the ancient and mysterious crypt and now is about to lie his hands on the object of his long quest and/or about to open the grim-looking gate leading to said object. But wait! As he approaches, a sudden roar chills the blood in his veins and a towering, nameless creature suddenly appears to stop him. It's the Guardian! Usually the monstrous/inhuman nature of the keeper is justified by its skills: monsters tend to be stronger and more resilient, with hard skin or maybe Combat Tentacles or enhaced senses or Breath Weapon or whatnot.