The Mysterious Letter is a cryptic letter given to Deckard Cain,[1] presumably by a Shadow. It is addressed to the Shard-seeker, and is also an invitation to learn about the Cycle of Strife, and after that join the Shadows.
Excerpt[]
When you stop looking for an answer, one is revealed. If you would strengthen Sanctuary against the outsiders, hurry to Selynne's Basilica and hear from the inheritors of the cycle. Their old stories have never been more true. They wait for you to become part of the telling.