Episode Description: Beware, spoilers! To read - hover over the text.
The Kingdom broke into the Black knight on honoring Prince Arthur. He was killed by king Uther, Lord Tristan De Bois, the brother of the deceased Queen. The dead man was brought back to life by the witch Nimue that wants to destroy the Kingdom, and most importantly the king himself. But Merlin learns from the book of magic that you can kill him with a sword, tempered in the fiery dragon's breath. And he is with Gwen.