This achievement is gained upon completing all the quests in Rain of Fear.

Mercenary of Shard's Landing
Partisan of Shard's Landing
Mercenary of East Wastes: Zeixshi-Kar's Awakening
Partisan of East Wastes: Zeixshi-Kar's Awakening
Mercenary of The Crystal Caverns: Fragment of Fear
Partisan of The Crystal Caverns: Fragment of Fear
Mercenary of Kael Drakkel: The King's Madness
Partisan of Kael Drakkel: The King's Madness
Mercenary of The Breeding Grounds
Partisan of The Breeding Grounds
Mercenary of Evantil, the Vile Oak
Partisan of Evantil, the Vile Oak
Mercenary of Grelleth's Palace, the Chateau of Filth
Partisan of Grelleth's Palace, the Chateau of Filth
Mercenary of the Chapterhouse of the Fallen
Partisan of the Chapterhouse of the Fallen
Mercenary of the Valley of King Xorbb
Partisan of the Valley of King Xorbb