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Mithril Coins. These coins are new store-purchased currency that's coming in Update 10 and will be used for a variety of in-game activities.
Basically, the use of Mithril Coins is Turbine's way of streamlining several different smaller store options into a flexible currency. These coins can be used as replacements for revive tomes, stablemaster's writs, daily quest resets, and fast travel to quest NPCs.