When was that added in? Cuz last time I heard anything about it, is the observer saying he was gonna add that to the game
After floof bosses and Kiyoko being freed, Keros offers it to you at the not so low price of your soul.
I think the only other requirement is that you talk with the den mother and matrons about all religious talk options. Eventually he'll approach you next time you walk on the path to the shrine.
And its VERY permanent. Only a few bits of your body are changeable after that
And you can go back to the shrine to "reset" back to normal Floof TF, as it's your "new normal".