Manannan's letter was a letter found inside of the magic chest in Abdul Alhazred's room.
To the Brothers and Sisters of the Black Cloak Society:
I am sure you all have questions about this mysterious voice speaking to you in dreams as I once had. Fear it not, friends,--it is the voice of your mighty leader who is now preparing to walk among us once more.
The time is drawing near. The prison that holds him is weaking and it is now that we must join forces and tip Fate in our favor for the triumphant return of the Black Cloak Society to its former glory!
We must not lower our guard, however. All signs seem to point to the return of the Silver Cloaks as well, but years ago we made ever effort and sacrifice required of us to set in motion events that will lead to our victory. Even now, I have in my grasp one of the prophesied children.
But we do not yet have everything we need. Now, more than ever, we must find the location of the Box. Sister Lolotte is sparing no efforts to find its rumored location in Tamir.
To the return of the Black Cloak Society!
May you all be in shadows,