Dear Princess Celestia,
The first thing you will note upon reading this letter is the unusually formal way I am addressing you. No doubt you think I am being silly, or playing some sort of prank. This is not the case. From this moment forth, I must use your title as though I were not a princess. This is because I'm not.
I quit.
There it is in a nutshell, Princess. I have, of course, written a formal letter of resignation, which I've copied and sent to everypony in Equestria. You should receive yours soon, provided that the postal service isn't stopped up for a few months with all the deliveries. Also, you'll probably have noticed by now that all my regalia remains, as I no longer hold a royal title and it is therefore not my right to wear it. However, I have taken my annual salary and all that I wish to keep of my personal belongings. Everything else can be donated to Twilight (she might appreciate it) or sent to some charity. Everything, that is, except for my old armor from being Nightmare Moon, which I want preserved in a glass case in the throne room or entrance hall, if possible. If not, put it in the tall astronomy tower. You'll find it in a hidden room behind that tapestry of an anvil on the third floor.
This brings me to my new occupation. Although I could live quite comfortably on what I have saved up, I didn't quit for nothing. I've decided to go back to doing what I used to do before we defeated Discord and became princesses: smithing. Even though it's not my special talent, I remember how to do it quite well. In fact, that's how I made my Nightmare Moon armor; that stuff doesn't come from pure magic, you know! (This is also the reason you'll find it whole and undamaged behind that tapestry; you remember it was completely shattered upon my second defeat, but I was able to mend it.)
Oh, and that reminds me. Just because I'm no longer a princess doesn't mean I'm going to give up my other job. You remain Princess of the Day, because I'm still going to raise the moon and create the night. I'm just not doing the princess stuff anymore. It is my special talent, after all, and I'm not giving that up. And I'd still be glad to play the hero anytime some disaster strikes that you don't want to deal with. Maybe I'll team up with Twilight and her friends, they might be glad to have another alicorn with them. I'm moving to Ponyville anyway, since the ponies there are really nice and are more or less used to having an alicorn around. Surely having two won't kill them, right?
... Oh, I hope it doesn't kill them.
Anyway, I'm going to open my smithy near Sweet Apple Acres. It's a nice scenic spot, close enough to the Everfree Forest that I can go visit our old castle anytime I need or want to. Maybe someday I'll fix it up and live there again, as I think that at least half of it still legally belongs to me. I'll check on that if I ever want it, and see... do you still want your half? I kind of destroyed it in the Nightmare Moon incident. Either way, I'll probably visit there often to read, so if you ever need to find me, check there. Actually, you might want to check my smithy first, since I'll be living and working there. You won't be able to miss it, as I'll be building it out of dark stone and decorating the outside with night-themed carvings and metal embellishments made of the same blue starsteel that I used on my Nightmare Moon armor. Then there'll be blue- flaming torches, and maybe some glowing gems in the walls... I'll have to have midnight blue or purple velvet curtains... probably an observatory on top of my house, too, and the house will be adjacent to the flat smithy... I'm getting sidetracked. The point is, you won't be able to miss it.
I think that's all for now, Princess. (Faust, it feels so weird to call you that!) I quit, I'm not sorry. Come visit me sometime, we'll have tea or something-- after all, we are still blood sisters. In case you haven't picked this up by now, I'm not giving up my Alicornhood (I don't even know if that's possible). I think I'll actually start looking for a spouse once I get settled in, since I should have considerably more time for that sort of thing-- I'll keep you posted about that, if you like. Other than that...
Goodbye!
Love,Princess Ex-princess Luna
Postscript: Don't even think about coming after me. I'm not going back.
Post-postscript: Don't send the guards after me, either.
Post-post-postscript: Or anyone else.
Luna paused, tapping her quill against her cheek. Was there anything she'd forgotten? Deciding there wasn't, she rolled up the scroll, sealed it, and wrote "For Celestia" on the front before setting it on her desk. Next, she pulled out a stack of parchment. In the biggest letters she could write (one per page), she elegantly scripted, "Dear Equestria: I quit. Sincerely, Luna." Binding it together with a ribbon and giggling, she quickly cast a few simple spells: one to copy the stack so that there was one for every pony in Equestria, and another to teleport it all to the post office, addressing each to an individual pony. Grinning, she checked to see that everything was in place, grabbed the saddlebags she'd prepared, and left her bedroom. She strolled merrily through the halls of Canterlot Castle until she'd reached the kitchen. Grabbing a quick snack, she opened the biggest window and flew out into the night, heading for a small, unassuming village just south of Canterlot. Ponyville had no idea what was about to hit it.
That morning dawned like any other. The sun rose beautifully and steadily, casting a golden light over the peaceful gardens and panicking halls of Canterlot Castle. In the throne room, a guard rushed up to the terrified Princess Celestia, carefully grasping a scroll in his mouth. "We searched her room again, Your Majesty! This may be the only clue as to where she is, but it's addressed to you." The princess nodded grimly, taking the scroll in her magical grasp. She hadn't read for three seconds before the bellowing cry issued across all of Equestria, screamed in the Royal Canterlot Voice:
"LUNA!!!"