Fluttershy, your presence is required at the Canterlot Royal Castle. A carriage is on it way as you read this. You see, Luna… she has not had a lover to lay with since her banishment and return. And you may not believe it, but I can tell that your body is best made for taking thick long cocks for hours on end. So, since I trust you, I am asking that you lay with my sister and relieve her of her centuries-long dry spell. Do not worry about pregnancy, as there is a temporary infertility spell that was cast when you opened this letter. This WILL be long and messy, but you will be well compensated.
PS- I must warn you, she's quite the power bottom. Appropriate, since she has quite the powerful bottom!