Location: End Game
Pre-Requirements:
Miss-Able: Potentially (Dialog Choice)
Proceed through the reverse King Crab Inn, a couple helpful points
- Go to basement, open/read book to get key
- Open cupboard, teleport through to fish room, take a fish off the wall (not item, won't have gear symbol) to open blue door
- Go down hallway, take the fork to the south to Astarte, click a candle to burn her body, loot ashes for a key
- Continue through and fight The Void
- Continue through to meet up with Leandra and Zandalor to fight Icara
When Icara is defeated you are given a dialog option to continue with Leandra's wishes and soul forge the two sisters together, choose this option - I am assuming that choosing to not soul forge them will void the achievement.