The Blood Moon Ascendance
This was a poem - a rather well known one.
"Beneath the veils of shadows, a truth concealed does hide, But tread with caution, seeker, for peril doth abide. A labyrinth of riddles, a maze of twisted lore, In whispers of the darkest night, the answer lies in store," the letter continued, its words seemingly pleading for salvation from an unknown adversary and rescue from a labyrinth where they had ventured or something like that. But that part could be ignored because it was mainly just a part of the poem. Probably whoever sent this must have been apprehensive of someone coming into possession of it and harming them due to the letter.