Morgause's Note

Morgause sent her first note to Morgana, telling her to meet in the forest outside the city, beside the marked tree trunk at sundown.

Trivia

 * Morgause tends to send messages to Morgana in rather unconventional ways throughout the series. In Series 3 she enchanted a mirror and sent it as a gift for her birthday and shapeshifted to an old woman in order to pass her an object in another episode.
 * 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.