The merging of tarot cards, stones and demons into weapons to enhance performance.
You'll need to complete Hohenheim's Embryo from Saint Germain in Shinjuku Babel (X20, Y19).
Ask Saint Germain about Occultism, and ensure you unlock that expertise before completing the quest. He'll request 30 Soul Shards: you can also bring him an Azoth Sword. If you bring him the sword, he will let you keep it, it isn't lost.