This achievement is obtainable in the Ashes of Malmouth expansion. It requires honored reputation with the Coven of Ugdenbog and access to the Fleshworks/Infestation (final dungeon of the expansion you have to finish the quest The Living Factory). In the north part of the first part of the dungeon (Infestation), you might have noticed a locked door called "Living flesh door". To be able to open this door, you need a quest from Seer Rugia in Coven's Refuge, called The Other You. Once you have this quest and enter the dungeon you have to kill Allostria behind the living flesh door to unlock this achievement. Note: I am my own master is the Ultimate version of this achievement.