The Artifact Merchant is where you can spend your Artifact Coins to purchase Artifact Fragments.
This Merchant is different from other merchants, he basically has unlimited supply and always all Artifacts available at the price of 1 Artifact Fragment per 400 Artifact Coins.
Artifact Coin 
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Artifact Coins are obtained through the Artifact Chest, Valkyrie's Favor, Special Events, and the Elemental Tournament Shop.