Overlord II

Once Dark Fay is your first mistress buy her all the stuff with the Dark Fay in it. Once you bought all the items there will be a cutscene with some heavy boucing bedsprings and curious minions.

Domination Path

This achievement is unlocked in the Private Quarters in Netherworld Tower, there you will have 2 important places to go, where the Mistresses are in a long corridor and you must press to make Dark Fay your first Mistress, and to the right of the corridor there is the room decorations where you must buy all things after making Dark Fay your first mistress, after buying all things there will be a scene and then at the end the achievement will be unlocked.