The Secret Triwizard Challenge

You say, "Lumos!" and wave your wand.
You can see what appears to be one of the racers being flown into the sky by a monitor. She looks ok, but she's wrapped in something white.
You wave your wand over the ground. White threads cover the ground of the entire field. They appear to be spiderwebs... which can only mean one thing. Something is in this clearing, and it's a giant-