Operation Babel: New Tokyo Legacy

After completing Abyss Tower, and defeating at least one 7H, you can return to the Core Floor where you previously visited (and defeated a 7H) and unlock a door. While in the Core Floor, head to X05 Y09 (you will need float to get here). There is a marker at this location where you can exchange Soul Tokens. You have to give Soul Tokens before you are able to exchange for anything. You have to select "Speak", select "Soul Rebirth", and then select "Exchange" in order to buy anything. Once you buy anything here, this unlocks.