For this achievement you'll need to occupy the city of Ancient City of Quintex and build the Sanctum of Quintex. This achievement can be obtained in both the Eye of the Vortex campaign and the Mortal Empires campaign.
In the Eye of the Vortex campaign the city lies in the Iron Peaks region, and in the Mortal Empires campaign in the Titan Peaks region, both part of Naggaroth.
Upgrade the city to lvl 5 and build the Catacombs of Quintex. Upgrade the buidling to the Sanctum of Quintex and the achievement should unlock.