Morgause's Note

In the episode The Fires of Idirsholas, Morgause sent Morgana a note telling her to meet in the forest outiside the city beside the marked tree.

Behind the Scenes
In the original script it is said that a hawk would give the letter that Morgause sent to Lady Morgana but it was never shown so it led the audience to believe it was Morgause's magic that delivered the letter.