Omamori is not just an amulet, but a crucial element for remixing with headphones to upgrade it to the next rarity.
To remix, one needs to get ready a pair of fully levelled headphones (level 15) and a specific type of Omamori. Omamori needs to be consumed and the pair of headphones is upgraded (subject to certain probability) to the next rarity with level reset to 1.
