Coin Administrator Nordin said that you should kill the Strigik in the Strigik village, and bring the beaks and the disposable Aether Cleaner if you wanted to get Mithril Coins. When you brought 25 Strigik beaks and Aether Cleaner, he said the cleansed Aether was intended for the Asmodians and gave you Mithril Coins as a reward.