Atlantis Coins can be gained as loot from encountering rare Orichalcum Sea Dragons during Atlantis Rises and from sources such as Wanted Missions and Rare Titan Loot. Similarly, the first completion of a Season 2 stage, on both difficulties, awards the player with Atlantis Coins.
Atlantis Coins are used for a free summon in the Atlantis Summon. You need 100 Atlantis Coins for one Atlantis Summon.
Please note - The Atlantis Summon is only active once per month during the Atlantis Rises Event. If you have over 100 Atlantis Coins and the Atlantis Rises Event is not active, don't worry, you will be able to use your Atlantis Coins the next time Atlantis Rises is active.