The Coin Administration Officer Godin said that you could obtain Mithril Coins if you brought over 25 Fanatics Parchments and a Disposable Aether Refining Device from Subterranea. You gathered companions around you and you went into the underground city, where you were able to obtain the Fanatics Parchments described by Godin and purchase the Disposable Aether Refining Device in the shop. Following this, you were told that the task was successful, and you received a reward.