The Amulet of the Mists is obtained from the Oracle Suun in Divine Path, after the completion of the final mission Imperial Sanctum. It can be traded for one of several unique items available from the collectors in the Divine Path.
A single player character is awarded the amulet the first time they speak with Suun in the Divine Path. Regardless of how many times that character speaks with him again, even if they complete the Imperial Sanctum mission again, they will not receive another Amulet. Note that the Amulet can be traded to other players, but the only way the recipient can obtain an item for it is to make it to the Divine Path by beating the final mission.