This achievement is gained upon completing all the quests in the House of Thule.

Mercenary of The Feerrott
Partisan of The Feerrott
Mercenary of the House of Thule
Partisan of the House of Thule
Mercenary of the House of Thule, Upper Floors
Partisan of the House of Thule, Upper Floors
Mercenary of The Grounds
Partisan of The Grounds
Mercenary of The Library
Partisan of The Library
Mercenary of The Well
Partisan of The Well
Mercenary of Erudin Burning
Partisan of Erudin Burning
Mercenary of Morell's Castle
Partisan of Morell's Castle
Mercenary of Sanctum Somnium
Partisan of Sanctum Somnium
Mercenary of Al'Kabor's Nightmare
Partisan of Al'Kabor's Nightmare
Mercenary of Miragul's Nightmare
Partisan of Miragul's Nightmare
Mercenary of Fear Itself
Partisan of Fear Itself