Merlin - The Crystal Cave

Merlin and Arthur are attacked while in the forest and Arthur is shot in the back with an arrow.  Just when all hope is lost, an old man appears to heal him.  He takes Merlin in to a cave made of crystal, where magic originated from.  Merlin sees visions of the future in the crystals and interprets what he sees as Morgana killing Uther with a knife.  When Arther gives Morgana the same knife as a gift for her birthday, Merlin knows Uther's life is in peril.  How will he save Uther from Morgana without revealing his own magic?