Despite the description, this is for fusing 30 sentiments together in Harmonia (Fuse Sentiments) than modifying them with Metanoia (Modify Sentiments). You can always fuse the lesser Sentiments you come across early on from acquiring Torments to purify and then leave them. Either way, this is cumulative so if you don't get this at the end of the story, you should be able to quickly pop this out with the amount of Sentiments given through the Ordeals.