The Stone Demons are two bad guys from the cult BBC game show Raven. Essentially carbon copies of the Demons from "The Last Stand" (in that they serve as walls the warriors need to destroy), they stand at the slalom known as "Millstone Towers", a challenge which is played as early on in the tournament as possible (because it involves everybody).
"Millstone Towers" gameplay
The Raven leads his six warriors to the slalom, where he explains that the Stone Demons block their path must be slain if their voyage on the tournament is to continue. The Stone Demons stand behind a lever, set high above the ground; if this lever is pulled, the team who does so first will release a magical draught which will slay their Demon.
However, the lever is so high up that the warriors can't pull it from the ground, no matter how high they jump. Therefore, they've got to manoeuver three giant millstones through the slalom gates. Simple enough, except for the facts it's two teams against one another and the poles of the slalom gates are topped with Dragon's Eggs (up to season 4) or Slumbering Skulls (season 5 onwards). If an Egg/Skull is knocked off the top of its pole, it'll release thick green fumes which will paralyse the unlucky team for a few seconds. This is not helped by the slalom gates only just about being big enough for the millstone, let alone the warriors, to pass through.
The millstones are so heavy that three warriors are needed to move each one from the starting point to the stacking post at the far end, so they can climb up and pull the lever first, slaying their Demon and leaving three gold Treasure Rings in their wake, for the warriors to collect as they run through the gate where the Demon was. Once one team have slain their Demon and gotten away with the gold, the other Demon will turn his "baleful gaze" upon the other team; ergo, they all lose a life each because they've lost the challenge.
The glow of the last team being eliminated, along with the glows of the first Demon being slain and the gas from Dragon's Eggs/Slumbering Skulls was all CGI.