The second chapter of the Book of Mercenaries will be going live tomorrow with Patch 20.4. Arriving June 3, we'll get to learn more about Xyrella, the Priest Mercenary, in her chapter which will reward us with a Priest pack.

Quote From Blizzard

The Draenei called it The Night of Falling Stars, when shards of fractured Light rained across Azeroth. Guided by a mysterious voice, the priest Xyrella now searches the Barrens for the glowing shards which the voice promises can save her daughter's life. The priest's quest will be explored in Book of Mercenaries Xyrella, launching June 3rd. Defeating all 8 bosses in this linear adventure will reward 1 Priest pack, containing only Priest cards from Standard!

Guff's Coming Too!

The third chapter of Book of Mercenaries will be arriving in just under 2 weeks with Guff Runetotem getting to tell his story. Like Xyrella, we'll be getting a free card pack for completing the adventure though we'll be getting a Druid pack for those efforts rather than Priest.

Quote From Blizzard

Guff Runetotem has always just wanted to help, but when "helping" means accidentally starting a treant rampage that nearly flattens Thunder Bluff, Guff vows to set things right. His quest for redemption leads him and his companions to the Wailing Caverns. What Guff finds inside will be revealed in Book of Mercenaries Guff, launching June 15th. Defeating all 8 bosses in this linear adventure will reward 1 Druid pack, containing only Druid cards from Standard!