For meeting every Chthonic God. You will need to purchase the prophecy table from the house contractor to be able to track/unlock this prophecy.
Odds are Tisiphone and Alecto will be the last two you need to meet, they started showing up for me around run 8 during the Tartarus boss fight, but I’ve heard that they might show up later, depending on what conversations you’ve had and relationship meters.
Just make sure you talk to everyone in the house after every single run, ESPECIALLY if they have a “!” above their heads. Depending on what keepsakes you want early game, Ive used all my nectar on the chthonic gods within the house including Megara, but I don’t know if this affects their showing up