Merlin: Season 2, Episode 12

2 - 12: The Fires of Idirsholas

When the fires of Idirsholas burn for the first time in 300 years, Gaius fears a sorcerer has reformed the deadly Knights of Medhir. Arthur and Merlin ride out to face the Knights and barely escape with their lives. Upon returning home, they find everyone in Camelot is under a strange enchantment