Cup of Life

The Cup of Life is a magical chalice blessed by many years of sorcery whereby is water is drunk from the cup the drinker will survive death. However another life must be given in return in order than the balance of the world be restored.

Merlin travels to the Isle of the Blessed in order that he may sacrifice his life to save Arthur's. Nimueh call upon the gods to rain, which then fills the chalice, and the water is taken to Arthur in order that his life be spared.

Behind the scenes
The Cup of Life is probably a nod to the legends of The Holy Grail affiliated with King Arthur and his knights. Although Christianity is never referenced in the series, there are frequent references to The Great Purge whereby Uther appears to have removed not only magic from the land but also the Old Religion itself.