A Herald of the New Age:
The Camelot Knights stumble across an eerie shrine deep in the forest, its chilling atmosphere betraying a dark history best left undisturbed. But when Elyan ignores Merlin’s warning, his actions spark a chain of events more frightening than anyone could possibly have imagined… Can a grievous wrong be put right? Or will past demons change the future forever?

http://merlin.wikia.com/wiki/Episode_4.10
http://cultfix.co.uk/merlin-a-herald-of-the-new-age-14968.htm
http://www.spoilertv.com/2011/11/merlin-episode-410-herald-of-new-age.html?utm_s ource=dlvr.it&utm_medium=tumblr