784: I'm Far More Powerful Than You Earthlings
Introduce a character who's clearly out of DragonBallZ... as a villain, naturally. This only works if your players are familiar with the show... but it should be enough to freak them out if they are.
Example: They're fighting an evil monk who's slowly getting worn down. When it looks like the PCs are going to win, the monk erupts in golden fire, his hair rises up and turns gold, and he suddenly looks a hell of a lot more confident. (I don't have the Epic book, but I imagine you'd need it to portray a Super Saiyan. Rocks break on their open eyes, you know. The weakest super saiyan is a walking nuclear war.)
Example: Introduce a short, smiling, feminine-looking man with white skin, horns, and purple armour. Give him a couple of henchmen, one of whom is bloated and pink, and covered in spikes, and the other of whom is beautiful, long-haired and completely green. If they still don't get it, transform Freezer (for it is he) into one of his innumberable higher (more powerful) forms. "He grows to ten feet tall, his armour breaks off, and his entire frame ripples with muscle. He still looks bored as you batter him with greatswords - to little effect."
Example: The PCs are sent after a pair of twins who've been causing havoc. One is a blonde girl, the other a dark-haired boy, both in their late teens. They're incredibly strong if the PCs catch them (Epic-level monks with a few levels of sorcerer thrown in for fireballs - and their magic is utterly undetectable, making tracking nearly impossible), but they're not the real problem. When a whole town is found empty, they realise a hideous, inhuman monster is hunting these twins to attain his full power. For real kicks, allow the monster to devour the twins (he's even stronger than they are, BEFORE he absorbs them and gains world-shattering strength).
Example: The PCs are attacked by old foes, who are inexplicably stronger. But they don't want to kill the heroes; they just energy drain the strongest fighter with a strange device and run away. The PCs discover that the energy is being taken to an evil wizard who wants to use it to free an immensely powerful demon. Worse still, the wizard is able to cast a spell that magnifies the evil in your heart, putting you under his control (and making you far more powerful). If the PCs are unusually thick, mention that the wizard is an alien creature, three feet tall and covered in wrinkled yellow skin, and make him yell "Boo!" at every opportunity.