Well, where did they get Fluffy from, after all?

Harry, Ron, Neville and Hermione ran from Peeves as they heard Filch running towards them. They ran and turned at the nearest door, and frantically tried to open it. When it refused to budge, they turned to run, except for Hermione, who pulled out her wand and shouted "Alohamora!" at the door, which then swung open easily. The four hurried into the corridor, slammed the door shut, and stood there breathing, trying to be as silent as possible as the caretaker argued with Peeves on the other side of the door. After they heard Filch stomp off, the sighed in relief, until Neville hesitantly said "Uh, what's that?"

Up the corridor, there was a huge black and brown lump on the floor that was moving up and down. As Neville spoke, the lump shifted and grew, becoming a massive dog with three heads that turned ponderously to stare at the children with a blank feral gaze. The left head twisted and stared, and then growled, a noise that filled the corridor, before all three heads suddenly started barking, deep blasts of sound that you could feel pounding through the air.

The four students scrabbled for the door, trying to get it open, but the door didn't budge. As the huge dog stalked down the corridor towards them, the four became more and more frantic, but simply ended up getting in the way of each other desperate attempts to pull the door open. Soon the giant hound was so close that they could feel the triple breath on their now frozen faces, as they stood petrified with fear. At Neville's unconscious whimper, one of the heads let its jaw gape open, and the entire dog prepared to lunge. Just as the dog lunged, a leaden, quiet, voice whispered from the other end of the hallway, "Stop."

The dog froze, its jaws on either side of Neville's head. The voice whispered again, "Come. Sit." The dog retreated to the other end of the passageway, and curled around the now revealed figure of a man. The man was tall, with long black hair and beard, and he was holding with one hand a dark metal helmet, that Harry thought looked like the ones from ancient greece. With his other hand, he began stroking the three-headed dog, as he looked down upon it with an almost expressionless face.

"So, this is where you got to. Who brought you here, Kerberos?"

And I couldn't see an ending to this bit. I may update it later, may not. Whatever. Also, I will be adding little bits talking about the mythical dudes mentioned.

Oh, and it's HADES, in case you didn't get it. Hades was called 'the unseen one', and he had a helmet that let him go about, well, unseen. He was also called PLUTON, or 'the wealthy one', as all wealth came from under the earth, either as food grown from the ground, thought to be drawing wealth from below, or mined from under the earth. He had quite a few other names, as the ancients preferred not to invoke him by speaking his name, so they spoke of him by nicknames.