S1·E13 The Way Back

Tone and Merlin share a dream that, during a battle, Tone falls through a tree and finds himself in a hospital where he sees his mother in a coma which it's believed his presence could help her recover from. Merlin realizes it was a true vision since he had another of the same tree, the new location of the Key of Truth for King Arthur - which can actually be used along with another to transport the knights home, if briefly. But the tree is inhabited by the giant Grinlap. Arthur, Tone, and Darren with his own key set out to retrieve it so Tone can see his mother awaken, but the Warlords don't intend to let them have another key...