The Quicksand Man is an evil chalk creature made of sand that can put people to sleep and settles into quicksand, to send them into his realm called the Nightmarerium.
The Nightmarerium is a dreaded place that manifests one's worst nightmares, and after a while, those trapped in it become mindless slaves.
After the Quicksand Man sent all the people of ChalkZone, including Rudy and Penny, to the Nightmarerium, Rudy escaped by dunking his head in cold water.
Rudy then went back to the real world to wake everyone up by scraping a chalkboard (the screech is amplified in ChalkZone), which was also the Quicksand Man's weakness.