The Priest-exclusive Trinket, Cassandra's Tome, has been found in Season of Discovery! This Trinket provides additional Shadow Damage, Healing, and an on-use effect that grants a 100% increased critical strike chance for your next non-periodic spell!
Cassandra's Tome Priest Class Trinket Guide In order to acquire this Trinket, a Priest will first need to loot Tarnished Bronze Scale from Chromaggus in Blackwing Lair.
Once equipped, the player will need to use this item in Tanaris, where they will be teleported to another plane of existence.
In this other world, you will be able to speak with a Dragon and peruse his books. However, once you return to Azeroth, you will need to obtain 5 Grimy Rune that can be dropped by level 55+ enemies anywhere in the world.