Thrall Demons was a challenge in Raven that was set indoors. In Series 1, it was played on Friday on the final week in Raven's Lair. In Series 2-5, it was played inside Wizard's Tower and was played very frequently in Series 2 and frequently in Series 5. It was not the only challenge played in Raven's Lair or Wizard's Tower, as there was also Tilt Maze (Series 1) in Raven's Lair, and Dragon's Blood (Series 2-5, also known as Wizard's Maze), and Gold Rush (Series 3) in Wizard's Tower.
Anyway, this article is about Thrall Demons, so let's not worry about any other challenges. In Thrall Demons, one warrior is selected by Raven to enter the Wizard's Tower and play the challenge (it's usually the warrior in 1st place). The warrior who is selected selects another warrior to aid them and tell them where to go/what to do. Both warriors then enter the Wizard's Tower.
When the challenge begins, the warrior chosen by Raven will be blindfolded in a dark room with thrall-threads with bells attached and demons. His/her goal is to avoid the bells from ringing and collect the 4 gold rings at the end, which will be shared. The other warrior says things like 'forward', 'left' and 'duck' so the blindfolded warrior knows where to go.
At the end of the challenge, if the blindfolded warrior reaches the end of the challenge, each warrior gets two gold rings. If a bell rings, then the blindfolded warrior will be taken by the demons, and lose a life. But the warrior selected to play alongside the leader does not lose a life if they fail.