Old Religion (Merlin)
The Old Religion was both a religion and magic that featured in BBC's Merlin.
In the time before King Uther Pendragon's age, the lands were filled with magical beasts and sorcerors who made use of magic which was termed as part of the Old Religion. However, after the death of Uthers wife due to magical means, the King of Camelot began the Great Purge which was an attempt at wiping out anything related with magic from the lands. Thus, the ways of the Old Religion became secret and close to dying.
The Great Dragon believes that Arthur was the key to restoring the ancient magics of the Old Religion to the lands. Thus, he charged Merlin with protecting the young prince and ensure that he became King in order to bring about a time that would allow the Great Dragon to escape his prison in the caves.
Nimueh stated that the power of the Old Religion centered around balance. Thus, the magics involved were capable of bringing the near dead back to life but this required payment in the form of another life's ending.
The Questing Beast was said to have been a powerful magical creature from the Old Religion and its sighting was a omen of doom.