Merlin - Season 1 Episode 11

While hunting, Arthur and Merlin see a unicorn. Despite Merlin's protests, Arthur kills the beautiful creature to bring the crass Uther its horn as an ornament. The foolish father and son ignore Gaius's warning that it is bad luck to kill a unicorn, but soon the crops wither and the well water turns to sand. The mystic Anhora, guardian of the unicorns, arrives in Camelot to tell Arthur that his stupidity has cursed the land and he alone can lift that curse. This involves his learning much-needed humility before riding to the Labyrinth of Gedref, where he will be tested. Merlin follows him but, by doing so, places both boys in danger. Unusually, it is Arthur who saves the day with a demonstration of selflessness, ultimately restoring the welfare of Camelot and the unicorn itself.