You are not alone.
Go to Riverwood and seek the guidance I have struggled to give you.
Go to the ruins near Rorikstead and in Winterhold's college and find equipment that will aid you.
Find the shards and fragments of our kingdom lost to time.
The Gossip Stones are the last vestiges of truth.
The fairies will guide you.
No one alive knows the whereabouts of the relics of Hyrule.
The Fairy Letter is delivered by the courier, but is done so "quietly." That is to say, the courier will not mention it, and the game will not display a message regarding its addition to the player's inventory.
The Fairy Letter refers to the location of the Forgotten Sword and chest in Riverwood containing the first hint to the location of the Mask of Truth. The Fairy Letter also refers to the Terminan Forge and Ancient Enchanter. It can be assumed the letter was written by the Last Great Fairy.
The Letter can be found carried by other characters, most of whom are dead, indicating that the player is not the only person the Great Fairy reached out to for the preservation of the relics of Hyrule, hence the phrase "you are not alone."
Background and Inspiration Edit
The Fairy Letter was added in version 6.3 of Relics of Hyrule as a means of directly guiding the player and providing a set starting point. This was a feature that was often requested, but JKalenad found it challenging to implement guidance without it being overbearing or immersion-breaking. The line "seek the guidance I have struggled to give you" comes from not only the Great Fairy, but also JKalenad himself.
The final line of the letter was included to emphasize that no familiar characters, such as Link, Zelda, Ganon, Midna, etc. are present in Relics of Hyrule, excluding the Hero's Shade.
The "quiet" delivery of the Fairy Letter, while unintentional, was not "fixed." This was done to allow the player to stumble across the letter in their inventory, keeping with the design of the mod.
The phrase "you are not alone" is a quote from the Doctor Who episode "Gridlock."