Book 4 Chapter 10
After you gain entry into Baba Yagas home, keep following purple orb until you meet the homeowners.
Listen to the conversation. The sisters will ask you reveal something of yourself something you are ashamed of, the only option which appeared for me was that I ran away from Nela during the bomb, however I assume if you made different choices throughout the game you will have other options. However the sisters view this memory as a classic example of human selfishness and self preservation.
Make sure that you feed the sisters this memory, it belongs to them now. You may have to feed the sisters another memory if they deem the first unworthy however I only gave the one about Nela.
Keep listening to the conversation eventually Zoe pleads for the item she requires. After promising to always remember them they give you the item and the achievement unlocks.